THX recommends putting the SBL and SBR together. as shown
here
Dolby recommends putting them apart like
this
Ya know, I believe different processing modes recommend different spacing. THX's Ultra recommends having the backs within 4 ft, IIRC, for example.
OTOH, if using most any other mode, place them as you would stereo mains up front. Heck I just came up with an idea. Hook up front/main amp channels to the rear speakers, and when you find the best stereo image with regards to stage, without a hole in the middle, imaging, and all that, then, well ... voila!
I guess if I am running a 5.1 with surrounds at 110 degrees and I add surround backs, then I will move my surrounds to 90 degrees and put the surround backs together.
Correct.
If I have two rows of seating, where should the normal surrounds be placed in a 7.1 setup? Do I split the difference and mount them between the two rows, and still mount the surround back speakers together as in the THX setup?
Enter Audio Video Philosophy 101.
Ok, kidding aside, extremely knowledgable folks do have differing philosophies. If you read up on the audioholics article
How To Calibrate A Multi Row Theater, you will note that they are compromising for all seats and rows.
I brought this issue up elsewhere, and others will tend towards calibrating for one ideal spot. For if you "compromise", NO ONE gets to enjoy ideal audio. I thought about this, and I fill up all 8 seats roughly once a month, as I host a movie club of sorts. Sometimes more often, sometimes less often. Anyways, I still decided to go for ideal audio up front/center, because by far most viewing is done with just a few people or less. See? Matter of philosophy.
OTOH, I am starting to look forward to trying some bipoles for the side surrounds eventually. I want to experiment, as I've never used this before, because I would enjoy a more immersive side surround with less of a hole between them and the fronts.
Oh, I even considered doing dedicated side surrounds for each row. Ya never know, might happen one day. As I use a receiver for a pre-pro, I actually do have 14 channels of amp disposable to me. Other improvements I could make would be to raise the rears higher to fire over back row, and I have a serious issue of a center too close to floor, sitting on an acoustical panel, but oh well. Win some, lose some.
Hope that helps.