What you need to understand is that these games you are playing in MAME™ are not PC remakes of classics, they are the real arcade
games being emulated on your PC. So if the game only had one set of
controls that you took turns playing at the arcade, the emulation
software cannot change that. Games that had 2-player simultaneous play
will let you do so on the X-Arcade™.
You can however
simply reprogram the 2nd player side's controls to the exact same
buttons as the 1st player side for these games, and then you could use
either side for playing. Keep in mind that the person not playing could
interfere with the other's turn. Click Here to see the layout for this.