mame menu, service menu, exit..
pause you could make player 1+2 start.
There is always going to be some problem when using the two player start buttons together to perform a function. In this case, it may only break up the action when both players are trying to bring their character back into the game. I am sure I am not alone here in saying that player 1 start + player 2 start should rarely (if ever) be used as a combo. IMHO, never use any "game buttons" when it comes to admin functions.
As per the original question, I also have a 3 button admin plan: Shift. Escape (only when shift is held). Pause. (colored Black, Red, Green respectivly)
EDIT: And yes.. I DO have a dedicated shift button... I find this works very well, also the fact I have to hold the shift button and tap the exit button prevents accidental exiting, although this may take a second to explain to people.