One coin button each or just one coin button?
I was always paranoid about there being some game that required each player to have their own, but I never really researched it.
*Sigh* . . . As I've posted at least 10 times in the last year

, Gauntlet, TMNT, Simpsons, and X-Men all assign credits based on the coin slot you use.
There are 2-player versions of these ROMs allowing the player to select which character they want to be, but credits are still assigned based on coin slot.
No P2 Coin = no two-player action on those games
unless you change the MAME settings so P2 Coin is also associated with the same keystroke/button as P1 Coin.
Personally, I like having dedicated P1/P2 Coin, P1/P2 Start, Esc and Pause buttons. YMMV.
Before you decide on shifted vs. dedicated buttons, consider reading one of the many threads that mention accidently triggering an undesired shifted function (exit, menu, etc.) while trying to coin-up during 2-player gameplay.
If you were about to play a game of football and you were told that there was a very small landmine hidden somewhere on the field you were about to use, would you still want to play?
Granted there's not much chance of stepping on it, but I'd prefer playing on a field without the booby trap.
Scott