Should be a quick one. I know you can mix AV channels to overlay images but if I connect 2 raspberry pi to one screen in AV and no multiplexer how will the channels mix if they are at different resolutions? This is not taking into account any input lag from the Pi of course.
You can’t combine 2 composite video outputs - you would just get a corrupted output. You would need some form of switcher to switch one or the other.