There will still be issues with 0.9 since it's not complete.
I haven't tested the multimouse setup yet but as far as I've understood it connects each mouse to it's specific screen when starting and running in fullscreen. If you're running in windowed mode then this is disabled to allow the mouse to pass between windows.
It's supposed to limit the mouse to not pass between screens in fullscreen as this would move the mouse input to a different zone. In fullscreen the configured mouse will only work on the configured screen.