Oh- basically you’d need to render your multiple cameras to different
subsections of a rendertexture, and serve that rendertexture to Syphon.
I’ve done this before because mad doubler is awful, but i don’t have a generic solution for the problem. If i were you, i’d just use a particular camera as the ‘rendered composition’ which serves to syphon, then render the other cameras to rendertextures that display as planes in your scenes.