the idea isn't radical, has been done with stacked advents, and dahlquists, probably others. I suspect the results you get from mixing 2 diff speakers will be un-predictable at best. But if it works when trying it, and doesn't create impedance issues, more power to ya.
Radical thought- Use 2 pair of speakers
Ok this sounds crazy at first but, is it possible and practical to hook up two sets of speakers for your mains. Each pair has it's own unique characteristics to why not add them together? It is still a 2 channel system. A bigger sound stage? If you have two matched amps, dedicate one to each? I know there are many reason why this cuts accross conventional logic but has anyone tested this theory?