If you were here a few years back, you could say it was the Keyclone forum, and not dual-boxing.
Whatever software is the strongest option, will be used by the most, and will end up talked about more often whether its questions on how to get it working or ideas on how to do something in the game with it.

You can two-box (dual-box) with IS Boxer, as with probably all of the other boxing programs.

I cannot speak for Lax, as to whether a Mac version would ever be made.
But you can go with... I think it's called Bootcamp.
And have windows on your Mac.
Which lets you use IS Boxer.
Not sure if that's the answer you're looking for.