Hi Sootshe,
Introducing too large vertical distances between VHF and UHF driver, You would introduce very large comb filter effects round UHF/UHF crossover region in the vertical plane.
I think it wold be better if You can introduce active (3-Way) DSP supported crossover in the system so time compensation can be applied, and mounting UHF driver almost in the front plate as near as possible to the VHF horn upper part of the horn mouth, or under the horn, mounted in the bass front baffle.
regards
ivica