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

I've seen people running two pairs of speakers, either from one amp or two amps, and enjoying the fullness of sound.
If it sounds good, why not?
Of course, 'good' depends on the personal taste.
I think some pairs may work, but there's a high chance that the sound gets muddy with interference if not properly set up. With one amp, at least the efficiency should be in similar range.

So I just got another pair of Focal Sopra 3 for another room but decided to try how it would sound to have 2 pairs running side by side in the same room with the same integrated amplifier (Anthem STR). Initially I placed them side by side and the sound was visceral and the dynamics were unbelievable. But the sound stage got muddy. Then I measured the tweeter distance from listening position on both pairs and got them aligned as good as I could so that the upper frequencies would reach my ear at the same time. The sound stage got much better and all the qualities of running both pairs at the same time were still intact. I must saw I am in complete awe. The sound is so much more enveloping, real, effortless. The bass is more even and full. I also got rid of some of the peaks from the standing waves, which could be a result of having both pairs in slightly different locations in the room. Absolutely Loving it. Now I’m not sure if want to take one of the pairs to the other room and just want to leave them both side by side. Will play with placement to get it aligned even better to get the sound stage to be as good as playing the single pair, if that is even possible. 
Having a lot of fun with this! 

Great! In the past I have only done large Advents stacked and some smaller stand mounts, also tweeter to tweeter. Now I'm playing my highly modified Klipsch Quartets and RF 3 lll together and enjoying the sound.

Bilaltata,  you will see on the first page of this thread the success I had (and still have) running Focal Utopias and Focal Electra 1038BE2’s together.  What I’ve  done since then to make them even better in performing as a pair is muting the tweeters on the Electra 1038 BE’s.  Two pairs of tweeters can compete with each other and creat comb filtering.  If you mute one pair, you may find the imaging and overall sound is even better, beating what you have already achieved.   

I have found that certain combinations of speaker pairs produce a better sound than the single pair alone



~~~~~~~~~~~~~~~~~
Yes I've been experiementing lately and found this to be 100% true.
1 speaker will not make magic happen.
My previous local tech had stacked Acoustic Reserach from the 1970's, sounded great
Richard Gray stacks the Realistic Rat Shak Nova 7B's. Sound great. 
I am stacking a  high sens 91db  FR with dual low sens 87 db Seas W18 midwoofer, xover at 2k, about to adda  2.2 cap and bring this down to 1600hz. 
The 5 inch FR whizzzer cone lacks the low end, the dual W18's come in beautifully.
Success at pairing low sens + relatively high sens.
Pairing dif speakers together is really the only way to go.
This is the new high fidelity.