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

Running 2 pairs of dissimilar speakers at the same time is such a bad idea, in a stereo system. Especially in the high frequencies.

Not sure if the OP is talking about stacking them, or playing them on 4 different walls, but either way, not good.

People, comb filtering is a thing. The frequency response would be completely unpredictable, there is no way to know if some frequencies are going to increase or decrease due to frequencies from one speaker’s dips or rises in frequency response meeting the other speaker’s dips and rises.

Then there are difference in lobing caused by different drivers and different crossovers. I am sure that imaging and soundstage would be close to nonexistent.

Check out the following video by speaker design wiz, Danny Richie. He explains why 2 tweeters on the same baffle is a bad idea, but a lot of what he says can be translated to multiple speakers of different designs also. Unless special circumstances are taken into consideration, like line arrays, for example.

https://www.youtube.com/watch?v=MGSfaKWcetQ

I would guess that any result that would sound ’good’ would be the result of certain frequencies, like maybe the presence region (4-6 K), being increased or decreased, depending on if the listener like bright or rolled off sound, or increase in bass response, etc.

Has anyone seen the Tekton "The perfect Set"? It has a few tweeters in each cabinet!

But if you read the white paper on the design behind this, it is not just sticking a few tweeters in the cabinet.

They are all acting as one driver. In other words, lobing, phase, cancellation, etc, are all taken into consideration by Eric Alexander in the placement and crossover design.

He has several very informative vids on YT explaining the reason for the design. And again, it is not just sticking a bunch of tweeters in a cabinet. 

I will reiterate what I said months ago, the chances of getting a good, flat frequency response, with phase not becoming any worse than one pair of the speakers being used, with no weird new, cancelations, standing waves, increased dips or bumps in the frequency response, or other unpredictable results, are not good. 

If one likes the result, that is fine.  Maybe you like whatever frequency aberrations are caused by multiple drivers operating at the same frequencies.