Most MAME two player games have the option (dip switches or setup menu) to use "common" coin slots, i.e. one coin slot gives credits for both players.
I've set up my two player cab with only one coin button, because out of the 1600 or so arcade games on my cab, maybe a handful didn't have the "common" option. And for those, you can always set up the same key for both coins. This way, if you press 5 (or whatever key you set), both players will get a credit. It's not elegant, but it works, and for me those few games didn't justify drilling another hole and wiring up another button.