You don't have to do anything in the Ipac/J-pac. Everything can be done in MAME.
Here is what I think your solution is.
Assign everything in MAME:
Credit for Player 1 Credit for Player 2
Push B1+1Start=Credit Push B2+2start=Credit
This setup is good for the games that requires to have separate credits (mostly in 4 player games).
Assign for EXIT
1start+2start=EXIT
Some people experience this setup to be a little troublesome in the heat of a game. Sometimes the two player hit thier start button at the same time. So maybe assign it like this....
1start+2start+p1b1 (or anything like Joy1-UP etc..) If you have pinball buttons on the side of the cp use those.
PINBALL B1=LSHIFT (located on left side of CP) + Pinball B2= RSHIFT(located on right side of CP). You can be really creative if you like, but NOT in the ENCODERS, do it all in MAME CONFIG.