For the sake of clarity (and to give @Sweets a day off) what you're referring to is actually a uni-compax, with two subdials. "Bi-compax" doesn't really exist - at least in the UG universe, where the terms "invented" - even if it's been kind of adopted within the modern horological glossary.
I think the precise nomenclature is as follows:
- Uni-compax = two subdials
- Compax = three subdials
- Tri-compax = four subdials
It's a bit confusing, and (for purposes of pedantry) I think that, technically, it actually refers to the number of complications as opposed to the number of subdials. Anyway, sorry to ramble on, but it's all quite interesting to a proper watch nerd.