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.