OK I now have a partial success.
Someone mentioned, that multiplayer via gamestream works, if a controller is physically connected to the host.
As I don't have a third controller, I thought, why not emulating one or more additionaly devices?
For that, I downloaded ScpVBus (virtual driver) and vXboxInterface (to attach virtual controllers).
With that, I can now play cemu in multiplayer over gamestream!!!!
And also, both controllers are detected in retroarch menu (I also can configure them seperatley). BUT while in game, only one controller is working. I hope I will find a solution...
If someone wahts to give it a try, here is the link for needed files:
click