In the genvdp.txt doc it says that the VDP could potentially have an overlay capability leftover from the TMS9918. So this thing crossed my mind in the past few days while messing with the VDP looking for unused bits/pins/crap etc. If that capability was indeed present, that would mean I could connect 2 megadrives together, or more generally, a megadrive and another system while sharing the same audio and video outputs. So yeah I did some work on it to see if it was possible and.. yes it is!
I had the idea of building a "MDX2", a system having 2 megadrive boards, 2 slots and 2 controller ports while sharing the same AV (they would be superimposed on each other). The idea is playing 2 single player games (same game or not) as it was multiplayer, maybe with a random timer to switch controllers electronically :V
A while ago I connected a Gen3 to an MD1, the MD1 acting as a slave device running a program made by me (that sets up the VDP for Sync input), but sadly it requires heavy mods on the system to pull it off (and maybe patching the games)
What you see here is 2 MD, 1 running S&K, the other running my program with their RGB lines connected together.
As I said before, the sync input should also work with other systems but I didn't test any other than this