This is something that is highly desirable and has been suggested for future version of Maximus Arcade.
I'm pretty sure it will be implemented in the new version 3.0 of Maximus Arcade, but that may be a long ways off. I sent Mameseer (the author) a quick note to see if he can implement this in the next beta 2.10 version. (We are currently at 2.10 beta 6). I'm thinking he may be able to add an additional "MAME2" in the dropdown list in preferences. That way you can simply point it to another directory with the older MAME in it.
Stay tuned.
~ DeLuSioNaL