SCART is indeed RGB as well as taking S-video and composite; RGB is switchable via voltage instead of being composite.
You could mod each console to output via a 9 core cable (plus shielding to use as grounding) for each system, all wired to the pins in one male SCART port. You'd then need to switch between each system via 5 x DPDT switches to make each system independent. You'd also need to make a box to house those switches.
That would be a cheap solution, if you're handy with soldering iron and ok to open consoles up to do work.
Would be an involving project if you did it, and i'd suggest if you did it, you register and post your progress on my forum so i'd be able to offer help along the way when you need it.
My solution, to this problem and also mounds of controllers, power supplies and many more systems; is my Unity system i'm working on, which is why I know what i'm talking about!