With current one, playing 2 player games is downright impossible unless you have also a gamepad. Well, if you don't want to play a game of Hand Twister at the same time as well, that is.

So I'm proposing following mappings for the 2nd player:

Left: Numpad4
Up: Numpad8
Down: Numpad5
Right: Numpad6
Btn1: Numpad7
Btn2: Numpad9

Then our friend can sit at the side and while we're using arrows+z/x, he'd use just one hand to control the game.

This is exactly how I've set up second player in Nestopia, aside of mapping stert/select to Num+/NumEnter and it works remarkably well.

Wrong, just wrong, please just read the documentation file, pico-8.txt. First player is [n][m] and arrows ([n][m] works as [z][x]), second player is [tab][q] and [e][s][d][f] for arrows. No hand twister needed with a little reading.

Not everybody has a Numpad. Zep has probably taken that into consideration when designing this control scheme, you have not. What works for you doesn't necessarily work for the majority.