This guy, aqwawaf, made an assisted speed run of four different Mario games running at the same time. Then, he posted the video on Youtube. This video uses four NES emulators all hooked up to a single controller input. The only way to get something like this accomplished is using emulators and save states to tweak Mario's every twitch. This is what's known as a tool-assisted speed run. It's technically possible for a human being to actually perform these movements, but the timing would have to be so perfected that it's virtually impossible. Luckily (?) we can manipulate emulators into doing all sorts of strange stuff these days. If you don't know your Mario games, he's running through two copies of Super Mario Bro's on the right hand side. The left hand side has Mario 3 and Mario 2 stacked top and bottom.
Watching this video makes my brain hurt. All I can hear are the cacophonous beeps and blips from four different Mario games going on all at the same time. My eyes keep snapping back and forth between the virtual screens and it's very difficult to keep up with what's going on in any single game. It's a chaotic sight, but watching it you can discern the differences between the control systems from game to game. I don't recommend turning up the volume too loud, it's a little overwhelming, but awesome.
It's a relatively long clip, running 13:27 on youtube. You don't really need to watch the whole thing to get what's going on. If you want to check out a demonstration of how synced up the author had to be, flip to about ten minutes in and watch him defeat the final bosses of all three games simultaneously. That's it, now watch the video.