Does anyone play two pairs of speakers at the same time?

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.

Pairs of dissimilar speakers? No. Pairs of matched speakers, sure. Double Advents are well known as are double pairs of KLH-9's. Double Quads are another well regarded option. Ever consider that Acoustat 2+2 are really doubled 1+1? Nobody thought that was odd that I remember.
Personally, I run two pair of VMPS Tower II at the same time and have for years. They're side by side thank you and sound wonderful.
Anytime you see "Purist" in this hobby watch out. That's the snob side of things leaking out over the experimenter side. There's no rule that says that two pairs of speakers, similar or dissimilar will sound one way or another. If you have such and want to play about, keep in mind that you might find something you like for little or no cost.
That's how discoveries are made.

"What a stupid idea to use two dissimilar speakers together at the same time! Goodbye coherence!"


Goodbye coherence, hello enjoyment.  

I switch to 7 channel stereo for casual listening at lower to mid-level volume, then to 2 channel for more critical listening at higher volume levels. Speakers are somewhat of a hodge podge at the moment with ESS AMT's for the front L/R, Martin Logan FX2's for the rear L/R, polk center and 2 velodyne subs (15" and 12")… Mostly used as home theater, but the ESS AMT's can rock pretty well in 2.1 channel.


I like the additional speakers at lower volume levels.

 

I wish one speaker set could do service to all the different quality and types of mastering out there. My audio collection is spread from the late 50’s to present. Many of these recordings have distortion, require mid bass enhancement and a “forgiving” tweeter. These qualities are better accomplished with a speaker design which has these qualities.

This same speaker will not deliver the tight bass and ultra clarity that better recordings offer. Another speaker with higher resolution and flatter response will deliver these goods better!

Hence, two speaker sets ( fronts)  could expand your appreciation to a broader range of music. Especially when you have contrasting types of speakers.

Comparing music playback quality is challenging for most people with their “music memory'. For me, it must be A-B immediate to really “see” the diff. After a couple years of using the two set speakers as either fully on or off, I acquired the Parasound P7. It offers a way to do it all. Set A, Set B or any individual volume of the both together. Tone sounds full and complete.

Have not noticed time delay issues or muddiness, though both speakers are precisely measured to ear distance. 

Just imagine Hendrix cranked up on ribbon tweeters and 6.5” woofers…. Could a vintage JBL sound better?

There may be some losses to this type of set up. It has been better for me.