http://www.makeuseof.com/tag/how-to-run-multiple-versions-of-the-same-program-on-your-pc/
It seems Galaxy once more is the problem here and you will need to petition GOG for a fix.
Running windows7 it simply will not allow you to open a second instance of GOG Galaxay, it seems the program checks itself even when you switch to a second user account (proving that if not for the program itself the second instance would be running).
Without this and with the hard coded matchmaking requiring you to link up to the GOG.com server list instead of doing a true search (you know as if the game was actually running on GOG servers or something *rolls eyes); a second instance on the same machine looks to be impossible.
Don't get me wrong you can get a second instance of Grim Dawn going no problem; you just cannot connect to galaxy for the internet list; making it completely pointless.
The only way to get around that personally is to find a server to play on, ask a player for the host to LAN it over service like Hamachi and use a NON-Galaxy instance to connect to that LAN game while you run an open online one to switch to.
But who the fucks gonna do that or be on board with it.
A more user based alternative is to make an advertised list on these forums around team speak groups based on +GMT.
So for say a 10am-11am GMT gaming lobby could be joined by people looking to game at that time; they could organise their LAN^IP client details & who is host then be playing.
The benefit to this is that times servers would remaining online would trend towards the advertised time slot;
People looking for multiplay would more naturally group then having to go find a mate then going fuck it i'll password protect my server (rolls eyes);
You don't have to rely on lengthy petitioning of industry dickheads who don't know how to run a matchmaking service to save their own platform;
Technically this would enable true crossplay between steamers and gogers because it would use 'neither' for connected multiplay;
Oh did i mention that being based around something like team speak would mean your actually able to chat while playing... arguably that's fairly useful.
But hey whatever i don't give a shit, this is just anouther fail for GOG.
Good work GOG we got you a decent multiplayer for your platform and you fucked up the attempt at being viable competition to steams platform... again.