Honestly, it depends on your choice of games. If you're playing games like 4player Pac-Man or Sprint 8, then you'll want the IPAC-4. If you only ever to expect two players (at most) to ever play at any one time, then use the IPAC-2 and wire the appropriate horizontal controls to appropriate vertical controls.
In other words, if the cab consists of panels 1, 2, 3, & 4. Then wire the controls of #3 in parallel to #1. The only catch is if, playing around someone mischievous, they can screw up player 1's game by manipulating panel #3. There are several solutions but the easiest is to add a manual switch to toggle between panel #1 and #3. Under that scheme panel #4 would be wired in parallel to panel #2. (Note, I am assuming panels #1, #2 is the vertical or head to head controls and #3 & #4 is the horizontal or side-by-side controls).