Radical thought- Use 2 pair of speakers

You are forgetting that benefits are not always additive. Sub woofers have long wavelengths, which are not directional to our ears, and hence can be additive. Mid range and tweeter frequencies are not additive....the result will be excessive comb filtering or a vague atmospheric sound that totally lacks focus. Even on a surround system the speakers should all match in timbre and characteristics (means you should really use the same speakers to get the most benefit).

Surround sound only works because the speakers are separated distinctly and the channel information fed to each speaker is specific for that arrangement, including delay adjustments according to specific speaker/listener positions.

Many recoriding studios use two different sets of monitors at once. If it works it works...

DV

DV, maybe that's why there are so many bad recordings..

I have done this multiple times over the years having been inspired by my friend's older brother's system(this is back in the 70's) that had matching Royal speakers stacked vertically. I remember he had a Scott tube amp and a AR turntable in separate room and this system rocked. I have had mostly negative results combining unmatched pairs.
When building multi-channel systems, it's recommended to have timbre matched speakers all around. The same speaker in each channel is considered ideal to create a "wall of sound". I now apply this approach and I'm currently using two pairs of Realistic Mimimus 7's stacked vertically and wired in series in my kitchen's two channel set up. With the extra pair, I have better bass. I have large speaker sound with a small speaker footprint.

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!