I'm going with the standard factory layout from mameroom.com for the control panels. I know 6 button is overkill, but who knows? Maybe I'll use it for something someday. Plus there are 3 buttons on the face panel below. Then I'm going to add my own 2 pinball buttons on the sides of each control panel for virtual pinball. I think that'll be a cool touch. So, that's 22 buttons total! Add that to the 8 contacts needed for the 2 joysticks, and that's 30 total. Too much for the minipac or the u360. The IPAC2 provides 32, so that'll work perfectly.
Wait, how does this add up, again?
Inputs - Used for
12 - 6 player buttons / player panel
8 - 4 joystick contacts / player panel
6 - 3 face panel buttons / player panel ***Functions? Coin, Start, ___?
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26 inputs
So far, so good. . . . then you get to 2 pinball flipper buttons / player panel
First - Not sure if anyone has a way to swap the table end-for-end for alternating play, so putting them on both P1 and P2 may be a huge waste.
If you insist on flippers for both players, wire the P1/P2 flippers in parallel with each other, since simultaneous head-to-head tables are pretty much unheard of.
Out of multiple thousands of tables, I've only seen
one simultaneous head-to-head table -- a mutant home-brew 3-player table.

(someone posted the video here a while back.)
Second - Left flipper is left shift, same as P1, Button 4.
Don't forget a ball launcher. (enter)
You also could add upper flippers (A, ') and Magnasaves (L-ctrl, R-ctrl)
Since no tables have both, you can wire upper flipper and Magnasave in parallel. (1 flipper for left, 1 for right)
Left upper flipper is A, same as P2, Button 1 -- requires adding a diode if upper flippers and Magnasave are wired in parallel.
Left Magnasave is left control, same as P1, Button 1 -- requires adding a diode if upper flippers and Magnasave are wired in parallel.
Nudge buttons L/R/Up (Z, /, space)
- Left nudge (Z) is the same as P1, Button 5
- Up nudge (space) is the same as P1, Button 3
So that gives us:
26 inputs - Buttons/joys/face panels
1 input - Right flipper (left is wired in parallel with P1B4)
1 input? - Ball launcher (assuming that none of the face panel inputs is "enter")
1 input - Right upper flipper (left is wired in parallel with P2B1)
1 input - Right Magnasave (left is wired in parallel with P1B1)
1 input - Right nudge (left is wired in parallel with P1B5, up is wired in parallel with P1B3)
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30-31 inputs if you use ALL of these options.
By my count, the Mini-Pac has 32 programmable button inputs, 2 mouse button inputs, and trackball/spinner inputs -- no problem.

Scott