There's any way to sync more than 1 Steam account on Galaxy 2.0?

No
And never will be
As GOG launches the steam game from your account, Unless you manually switch accounts then yes, otherwise no

This makes ZERO sense, why wouldn't the be able to prompt you for your username and password if you needed to switch?

as i said
"unless you manually switch accounts"
its just not worth the hassle

One way around this would be if Galaxy somehow could also import Steam Family Sharing games.
That way you can family share between your Steam accounts and have them both on Galaxy.
Really hoping that'll come at some point.

A hassle of a few seconds?

you keep forgetting about the devs mate
steam has its ToS and guidelines

This is incorrect, you wouldn't need to switch accounts at all.

It is 100% doable, basically every other frontend/launcher out there can do it and I'll explain exactly how it all works.

Let's use LaunchBox (one of the most popular frontends) as an example, an application developed entirely by one person.
You can scan however many Steam accounts that you own/want to, all of the games will show up in your LaunchBox library, and as long as you have access to them on the account you're logged in to in the Steam application with (through either owning them or family sharing), you can launch them.
All Launchbox needs is the URL for your different Steam accounts, or you scanning your executables, it's super basic.
It would work the exact same way in Galaxy 2, it's just an extension of Steam itself. That's not how the Steam integration work, you don't actually need to be logged in to anything other than the Steam application itself to open a Steam game, launchers and frontends don't "launch the games from our accounts", they send a command to Steam that Steam then handles.
You can add any game manually and link the executable to launch it without ever logging in to Steam in Galaxy 2, it's one of the first things I tested and this is how I've added my family shared games from my second Steam account to Galaxy 2.

There are two ways a Steam integration work in any launcher/frontend:
1: The application scans your computer locally for executables and launches the game by opening the executable themselves (which in turn launches Steam).
2: The application scans your profile for games and launches the games by sending a command to Steam.

It's a very simple command, you can do the same thing yourself by opening any web browser and write "http ://steam ://run/GAMEID]" (remove the spaces) to launch any game installed on your machine, whether you own it or have access to it through family sharing.
The web browser has no idea what account you are logged in to or what games you own, all it does it send a launch command to Steam and Steam handles the rest (checking for owned/family shared games).

Galaxy 2 has integrated a weird/interesting mix between both of them for user convenience, but there's nothing different or special about the way Galaxy 2 handles Steam game launches or Steam accounts.

You don't need to be logged in to Steam any other place than the Steam application itself for either options to work, it's just the way GOG has chosen to integrate it into Galaxy 2 so that all of the current and upcoming features will work out of the box (chat, etc.), without users having to fiddle with any settings or code themselves, but they could've just as well had done it the same way Launchbox and every other frontend/launcher has done it so far, it's just, as I said, for user convenience.

You could even make a new Steam integration for a second account to scan your games instead of adding them manually, all you need to do is target the Steam URL for the account that you want to import the games from, the "problem" here is that it would only work for the specific account that you made it for, so you would have to make one for every Steam account you own.
To get around that you could make an integration that scanned a specific path for executables, which is the option that the RetroArch integration is based on, all it does is link to your rom files and emulators, mark the games as owned in Galaxy 2 and link the executable, instead of having to do it manually.

The only reason that you "need to" log in to Steam in Galaxy 2 is to use the current friend integration and upcoming chat feature.
Games, achievements, and game-time scans can also be done without logging in, which is how websites like exophase work, again, all they need is your account URL, so you would have to base your integration on the same system that exophase uses.

And just to be clear, you can even get your Steam friends online status (but not the upcoming chat function) and other stuff like that by getting your own account API key (steamcommunity.com/dev/apikey), the main difference here is that GOG is doing it community-wide (I'm guessing that they've either requested a different kind of API from Valve or automatically get your personal API key when you log in through Galaxy 2) so that they don't have to add every Steam user manually, while if you did it yourself it would be per account, so other people wouldn't be able to use your integration files, only the specific account that you made it for would work.

So to sum it all up: yes, it is possible.

No offense, but please refrain from spreading misinformation in regards to something that you obviously know nothing about. Thank you :)

EDIT: And just to clarify, I don't mean the last part in a douchebag kind of way, I'm just annoyed at the fact that there's a tendency on the internet for a lot of people to give out wrong answers left and right, because of them not knowing about it and just taking a wild guess.

"GOG GALAXY 2.0 is importing information from your account from connected platforms and shows all the games available there, no matter if you have them installed or not. If you own games beyond platforms available in the app, you can add single titles manually to your master games collection."

"Do I need to have gaming clients installed on my PC to launch games via GOG GALAXY 2.0?
Yes, if you want to get access to any features specific to that platform, e.g. installing or auto-updating games, cloud saves, etc."
aka logged in.

See the attached image.

@FlowerBringer88
That's amazing, I hadn't though of that, thanks!:)

Sorry to bump the thread, considering it hasn't been touched in over 6 months now but I just read through the entire post and have deemed that you guys don't really know much about Steam or GOG Galaxy 2.0 except for @FlowBringer88.

First things first, being able to open games from both accounts. Let's simplify that further: Being able to switch accounts, without logging in/out. Impossible you might say, after-all Steam requires 2FA every time you log in. This is untrue. Let me ask you, when do you NOT need 2FA? When you open steam after pressing 'Remember me'. This saves your information, and even if you change your IP, change your computer name or even DNS, it will not disable or interrupt this option. The only way to do this is by manually pressing log-out (or switch account), by changing (spoofing) your HWID or more commonly, cleaning your cache.

That leads us back to the question. How do you actually 'Switch accounts' without re-logging in? Well, the answer has been there for a really long time. I have 5/6 Steam accounts now, don't ask why. And I ended up researching this back when I only had 3 accounts back in 2018. There are third party applications that allow you to log in to steam and have the ability to switch your accounts without re-logging in every time. They are 'Steam Account Switchers'. You can even make your own 'account switcher' by using a BATCH script and Steam shortcuts. The shortcuts hold a specific set of instructions (primarily holding onto the cache that usually holds the 'Remember me" information) and depending on how you look at it, are actually a very serious vulnerability in Steams design that hasn't been fixed in over 6 years, so I doubt it's gonna get fixed anytime soon unless they want to completely redesign Steam and the way it holds information for 'Remember Me'. It's too much time & effort and therefore money.

There are two prime examples of these switchers, and I use one of them. Both of these are NOT malicious as I have been using them for an extended period of 'A couple of years'. My steam accounts have NOT been hijacked nor has my information been stolen. I'm not familiar with the rules of this forum so I am unsure If I am allowed to post links so I'll have to do with just names and authors.

1) Steam Account Manager by ohmygosh. Originally made on Windows 7 if that's any indication and waas made in 2014. And still works. It works by storing what is called Connect Cache Tokens, as your 'Remember Me' is stored in the cache. Hence why when you clean your Cache you have to re-log. It is available on a particular (unknown) cheat forum. You have my word that this works flawlessly.

2) SAM - Steam Account Manager by rex706 on github. Last updated on the 3rd of May this year. Has a more 'Metro' design that's fitting of Windows 10. If you have ever used Depressurizer for the Steam Library then it has a similar vibe.
It requires a Steam API key if you want certain features such as tracking account bans, vac bans, time-outs. I am unsure if this uses the same method as the first account where it keeps it's own copy of the cache creating on Remember. However this version has far more utility. It allows you to set custom steam parameters when launching so that's not nothing new from the other one. However! Where it sets apart is the encryption of user-data and passwords. I think this is incredibly important.

Conclusion and/or TL:DR;
For those who said it is impossible, you clearly don't know shit nor have you researched any of this. The steam plugin in GOG uses Python. If you think you can't make something as simple as saving cache using Python: then you once again don't know shit. The biggest issue in this won't be switching accounts, but knowing WHEN to switch accounts and for which games. I'm sure if GOG made their own plugin and integration for this it would definitely be manageable. But just like most of you I don't know shit about Python or programming in-general. The most I've done is make a simple 2D game using C# in Unity.

Edit: As of today Friday 02/10/20 (DD/MM/YY) there are 6 different repos on GitHub for Steam Account management and switching, all of which can bypass the mandatory 2FA. Some of them use Python, some of them use .NET Framework 4.5 (minimum), some of them are pure executables and there is even one written in C# (.cs). Some of them have encryption on data, some of them don't. If you want to find them yourselves go on GitHub and search using these tags: steam-account-switcher, steam-account-manager, steam-accounts. There will be lots of results and you can check them out yourself with throw-away steam accounts if you don't want to risk yours.

DId anyone ever find a way to make this possible?

For various stupid reasons of my own, I have 2 Steam accounts. Ideally, I'd just merge the 2 accounts in Steam, but no. Obtuse Steam won't permit that. I've shared the libraries, which seems as helpful as Steam gets, but that doesn't help in GOG.

For another stupid reason of my own, I just managed to add a game to the wrong account, and so it won't show up in GOG.

It'd be great if GOG could help me from the failings of Steam (and my own stupidity).

this is stupid as fuck since it's possible to create a script that auto closes and opens your steam account and switches it to the one desired.

here it is-

$autoLoginUsername = "username"

# Only process if current saved username is not the same one
if ($autoLoginUsername -ne (Get-ItemProperty -Path "HKCU:\Software\Valve\Steam" -Name "AutoLoginUser").AutoLoginUser)
{
if ($null -ne (Get-Process | Where-Object {$_.Name -eq "steam"}))
{
# Gracefully shutdown steam if detected as running. Expects default location.
Start-Process "C:\Program Files (x86)\Steam\steam.exe" "-shutdown"
Start-Sleep -Milliseconds 1200
}

Set-ItemProperty -Path "HKCU:\Software\Valve\Steam" -Name "AutoLoginUser" -Value $autoLoginUsername
Set-ItemProperty -Path "HKCU:\Software\Valve\Steam" -Name "RememberPassword" -Value 1
}