Stacked Quads were/are(?) popular. And multi sub woofers are used. But outside of surround sound and home theatre, I don't personally know of anyone mixing different speakers simultaneously .
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.