Pretty sure its a DIP switch setting. So its the same ROM file - you just have to set the cabinet type too. I think TMNT and X-Men work this way too?
The games listed in this thread that do have dip switches don't have a 2-player option -- you need to run the child ROM.
No DIP switches in Simpsons -- "
simpsons2p" is the child ROM (contains only the ROMS that are different from the parent) and "simpsons" is the parent ROM. (contains the ROMs required for the default 4-player version of the game)
When you run the child ROM, it will access the common files from the parent ROM.
The child ROMs listed in my previous post also have parent ROMs that you will need to have in the "roms" folder. (gauntlet, gaunt2, tmnt, tmnt2, and xmen)
Scott