if you have two, why not do one for tree frogs and the other for a more arboreal dart, like d. leucomelas? since tree frogs like height, 7 feet would be ideal for them. the leucs would also love the extra climbing space and would probably dig out on 7 feet of height.

as far as lighting, a decent 24" wide t5ho light should give you pretty ample lighting to reach 7 feet down, provided you plant shade loving plants at the bottom.

should be a pretty cool conversion, i look forward to seeing how it works out.