Can you not just set the crossover point such that the signal rolls off just before the hump which combined with the hump gives relatively flat total response until the back side of the hump where the total rolloff is now steeper than 12db/oct? You would have to design the system for a steeper rolloff total on the back side of the hump.
(Forgive me is I'm way off base. I'm still rather new at this.)