With a 2 Player IPAC, your only going to be able to use up to 20 Buttons, including 1 and 2 player, so only 18 regular buttons and 1 each of your player buttons...
You see, the 2 Player IPAC had 28 Inputs, 8 of which are reserved for up, down, left, right on both 1 and 2 player... thus only 20 are left for arbitrary uses...
This means that If you get 23 buttons. You are either going to have to hack something else, or not use those 5 spare buttons, that or keeps them just for that reason, spares...