Everything is dissimilar. The "unified theory" is still yet to be proven. MOST speakers (in an enclosure) are assembled from "off-the -shelf' drivers from the few manufacturers who mass produce them. Who defines (and decides) what is "similar?" Virtually most speakers enclosures with their drivers that are commercially available aren't designed to be assimilated into speaker "X." I've heard systems far into stratospheric price levels with 2 sets of top-drawer speakers driven reference components (every piece $10,000 and above) that were highly satisfying. Paul McGowan of PS Audio has more than once lamented about the arrogant, opinionated and narrow-minded rantings of some in our hobby. You can disagree and still be polite and civil.
Does anyone play two pairs of speakers at the same time?
I have found that certain combinations of speaker pairs produce a better sound than the single pair alone. For example: Klipsch Quartets and PSB Image 4T (new tweeters from Vifa) Quartets inside pair and volume matched to PSBs. I have done this over the years and found some great combinations.