you can do it like this.
wire the ONE COIN input to lets say key 5.
MAP MAME Player 1 Coin input to Player 1 Button 1 AND 5 (so if you want the credit to Player 1, press/hold B1 player 1 and insert coin(5))
MAP MAME Player 2 Coin input to Player 2 Button 1 AND 5 (so if you want the credit to Player 2, press/hold B1 player 2 and insert coin(5))
This will work good for GAMES that require separated CREDIT for each player.
All other games it don't matter who puts the coin in where.
What games are you trying this with ?