The old, OLD, "Sweet 16" speaker project from Popular Electronics used 16 cheap 5" speakers on each side (but just one box per side), the theory being that crummy speakers driven very little together might produce good sound. My personal opinion is that Bose stole this idea and made a mint. I agree with the posts that say try different speakers, not two pair of the same. Let them 'average out' the sound. Plus the greater power capacity you get with more speakers. Don't spend big bucks, experiment first. I think what will suffer, from what I've heard using multiple speakers, is imaging. Good luck!
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?