Hi Mattis,
Currently, split screen with Suimono is problematic, due to the way it handles reflection and transparency. It isn't really possible to get both cameras to look correct unfortunately. Fixing this and having better multi-camera support (including split screen) is a goal I have for Suimono 3, which is currently in development.
Splitscreen in Tenkoku should be possible, as long as the two cameras aren't too far away from each other. By default Tenkoku tracks one camera, and adds the relevant cameraFX components to that camera when the scene starts playing. However, you can add the Tenkoku camera FX to both of your cameras manually, and they should then both be seeing proper sky rendering. The main issue here is when you start to render the rain or snow effects, as these are local to the tracked camera position. I'll be looking at better multi-camera support for Tenkoku once I settle on the final improvements to Suimono.