if you are just looking at 2 players, you can hook up a "shift" button.
the common would be from your coin drop line from the door and out from your NC (p1) and NO (p2) leads would be your player 1 and player 2 coinup, going to your ipac.
that way you just drop a coin to credit player 1 and hold the button and drop a coin to credit player 2.
This would be the best way I could think of to do it (other than using the default shifted IPAC keys of P1 start + P1 button 1 = coin 1, and P1 start + P2 button 1 = Coin 2)
I'm liking the idea of this, but from a wiring point of view I'm having trouble getting my head around it. I'm happy to do whatever providing I can trigger a coin in for player 1 and 2 independently.
If I got players to hold the P1 start (1) and set the ipac to issue a player 1 coin if the player one coin (5) button was triggered in combination with the P1 button being held - so 1 & 5 = 5, then do the same for player 2 start so 2 & 5 = 6? Would that work and is it even possible?
Would I not get an issue whereby a p1 game would start right away because the key press triggered by the coin will only be momentary? So the ipac would get 1&5=5(p1 coin) then player one start (1) right away?
Sorry for all the dumb questions, I have a limited amount of time to put this together so it's a challenge at the moment!!!
Edit - can you not change how games such as the Simpsons accept coins with the mame system menu?
Edit2 - I've done some testing and by the looks of it I'll be fine with just a single coin mech, if you run the 2 player rom of the Simpsons and TMNT credit in on slot 1 gives player 2 the ability to start a game and select a character.