I've used the same set up for dual-boxing while playing WoW. The snag I ran into was some type of latency.

I would start both characters (mages) off at pt A (right on top of one another). I had their primary spells/abilities mapped to the same spots/keys.
I hit 1... both would cast frost or fire spell.
I hit 2... both would cast spell ...

Anyway, the issue came into play when I try to move. They begin running in diff directions.
I guess this problem is solved via a follow macro and an assist macro?

If I have player b on follow, however, then my issue is that when I begin to cast... he's not in range, as he's behind player a (leader).

This hardware solution certainly works, but it raised those issues for me.