I think this is a terrific idea, and have been thinking about it for a while. It seems like it would lessen the stress on the owner with lots less trimming, etc. on the whole 8ft. Also, you can keep more light-sensitive fish (discus, rams?) and let them hang out in the dark 1/2!
With 2 halides, you have probably about 4 feet of high light area. So like you said, it's about 1/2 high light and 1/2 lower.
Do you plan on co2 injection? I think you should, with those lights they are intense. And if you do, the wet/dry filter will degass all the co2 so you will want to convert it to a sump only.
Your nutrient uptake will still be fast too, so you will need to keep an eye on fert levels. It just won't be AS fast as 8 feet of high light!!!!
If I were you, I might put the 2 halides in the center of the 8 feet. That way, you have lower light on either side and utilize any stray light rays that might have escaped the tank otherwise. ON the edges you could have open sand substrate with wood overhang, moss, anubias, maybe crypt/vallis background too. Sounds cool!
What's the intended fish population?