Once upon a time desktop PCs had a 2D graphics card and a 3D graphics card. A pass through cable connected the 2D into the 3D card, which combined and outputted the two signals over layed. If I had, say, two outputs, both the composite signal for old analogue TV, can you just combine the two signals together and, as long as they don't draw over the same location, get the combined output? Timing is crucial, obviously, they both have to send the vblank at the same time. I guess in the desktop PC days they would read the timing off of the 2D signal and use that to time the 3D signal.
Can you just sum the two signals? Or will you have to attenuate by 50% first?