That is not totally correct. I have successfully calibrated 3 subs in my room. Most Audyssey versions, as well as offerings from Pioneer and Yammy's only have the ability to calibrate one sub. However, that does not mean you can't do a great job and get the room sounding awesome.
Basically you first want to calibrate each sub manually, making sure they are the exact distance/position away from the primary first measurement location. My 350's are within .5" of distance from the primary seat, where I took the first reading. Next, you have to manually run the dB test tones and calibrate each sub independently, one by one, to the exact same SPL in dB's, I think I used 78dB's. Keep in mind, when they are combined, the dB's will increase by about 3dB's, so you might want to measure them a little lower, so when combined they will be where you want them.
In my case, I run Audyssey just on my 2 350's, which are up front, spread out between my M80's from all 8 locations. I also initially calibrated my EP600 to the same dB's as the 350's and it is placed in the back of the room.
I've tried including the EP600 when running Audyssey, but I get better results, just adding it after the fact and run Audyssey just against the 350's. The internal DSP of the 600 makes the distance settings difficult, normally it comes back at 25ft, when actually 8ft. This is normal for DSP subs, but when the 350's are 9ft away things get crazy. Averaging distance does not help either.
Anyway, yes can get fantastic results with a receiver and splitting out the subs. Another option is to just daisy chain them from sub to sub.
Receiver line out to Sub 1 line in, Sub 1 line out to Sub 2 line in. That is if your subs have dual RCA jacks.
