With an IPAC, you can assign one button as a shift key and then press the shift key and another key to get a shifted function. The reason for this is so that you don't have to have a ton of other buttons on your panel, allowing it to look more authentic.
What I'm planning on doing for mine is having 7 buttons for each player, along with player 1 and 2 starts. Then I'll have a shift button hidden away on each side of the control panel box. Shift+1P will be coin for 1P, Shift+2P will be coin for 2P, Shift+Button7 will probably be Exit game, etc. I'm planning on hooking up a coin door and using either quarters or tokens most of the time for that authentic feel, so this should work fine, I think.