One thing I'd be careful of is mixing drivers that were vastly different in displacement. And when I say displacement, I mean absolute displacement which includes power and efficiency as well as size and excursion capacity. What I wouldn't want is a large driver that could provide a lot of pressure combined in the same chamber as a small driver with less excursion capacity. The larger one would tend to push the smaller one beyond its limits.

I realize that you're talking open baffles, so some of these things will not apply. But you might consider the tuning aspect and coupling between drivers as you proceed.