More than one active camera?

So I have a problem with a Splitscreen script. The script works in my normal scene, but I now grouped the players and the cameras, to use them in other scenes with group instancing. Now if I instance both players in another scene, only one camera works with the splitscreen, the active camera. Apart from that everything works fine. If I change the active camera in the script to be the other camera, only the other camera works. My idea was, to make both cameras active, if that is possible.
Here´s the blend. Would be cool if I could get this to work, because then the game will work asset based and it will be easy to add new maps.

can you use 2 planes and render a video texture from the two cameras to the planes?

Typically you tell the camera to render in a viewport:

Sorry I can’t access your demo file

Wouldn’t render to texture reduce the quality?

This depends on the resolution of the texture you use for render to texture (and probably also the script)