If I'm not mistaken, the check order is:

Dev mode? 0x10fe00 - 0x10fe01 (probably due tu dev being made & debugged on home systems)
Retail MVS? 0xd00034 - 0xd00035
Home (last case) is wherever you put your own counter.


Also account for difference is US and jp/euro modes.

US: display 2 credit counters, pressing 2P start starts a single player game on 2P side
JP/Euro: singe credit counter, pressing 2P starts a 2 player game.