Had Elacs B6 running along klipsch p37s side by side, the sound was more "filling" but Soundstage was gone, reflections everywhere, and this was a rectangular high ceiling room, personally didn't like it, I moved to a new house and my room is a "difficult" challenging one, got audiokinesis Azels speakers and swarm which will make for better sound on the difficult rooms as you can adjust glare etc. Although I would like trying running other set of speakers just for fun and tinkering I am almost positive I won't like the sound. Note I am not criticizing experimentation as a matter of fact this is a very interesting topic, thanks to the OP.
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.