In the beginning, i wasn't thinking, that i have to install the client for that feature. And now, since i did, i'm wondering how to play spontaneously Online Co-Op or else. Is it only possible with peoples from my friendslist, or how i'm gonna do that; can you help me please? On the other side, i thought, that Co-Op would also be possible in Offline-Mode and as Local Multiplayer. So i hope, this feature is coming soon with an update, if nothing like that already exists! CU

Local coop is coming soon, you can already enable a unfinished version. Quoted from Myran: "To start a game in split screen open up the console (~ key) before starting a game and enter the command "g_start_sessions 2" (without the "s), then enter singleplayer menu and start a new game.
There are several issues with playing like this that we still need to resolve, but if you're feeling adventurous you can give it a try."

Hey; not bad for the beginning and better than nothing, thanks for the clue. I'm still wondering, if it depends on the game or maybe on gog-galaxy, because it's in beta-status. Oh and i forgot to explain, that i think it would be the best opinion to play local without splitscreen, or at least to choose between these two options. I don't like playing in splitscreen, because the display gets too tiny and it deflects each other.

Has the local coop been added since this information was posted?

Thank you for providing the command to activate the unfinished version via the console.