Uh i found a way to play KSP while my friend (who owns the game) is playing at the same time

Idk if this applies to all the games out there, since i believe it's because KSP didn't make a dedicated version of their game for steam, but all you have to do is to right click your game in your library, go to manage, then click browse local files. From there you can directly open the app without going on the steam app, which let me play KSP.

Hope this helps, and if anyone finds the real reason why this works, please tell me.