Each pair has (its) own unique characteristics to why not add them togetherBecause most likely you'll have more of cancellation and addition than complementarity, resulting in nulls & humps.
But... you never know!
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?