OK, it can be hard enough to have the coordination to play one game well. But two, simultaneously, with just one gamepad? At this year's Games Done Quick, Zach "zallard1" Allard not only clocked Mike Tyson's Punch-Out!!, but Super Punch-Out in under 24 minutes in these exact circumstances.
You can watch the whole thing in the video above, but if you're just curious about Allard's technique -- he doesn't chaotically press buttons, instead, he makes smart use of pauses and delays so he can switch his focus between the games.
It's also mentioned at the start of the clip that he makes use of audio cues, which is why he's not wearing a microphone / headset.
I can't imagine how much practise it would take to get this good. Maybe the challenge needs to be increased for the next attempt -- inputs via telekinesis only?