Go for it!! I went 8.2 with 2 sets of ProMedia 4.1s and now have a 9.2 set up (effectively 7.1). If you're debating whether or not going with the Z560s or the PM 4.1s and you aren't considering the PM 5.1s, go ahead and do the 2 X Z560s.
All you need are 2 1/8" stereo Y-jack adaptors.
EDIT: No sub cancellation will occur. I have had a 12.3 set up with 3 sets of PM 4.1s for a day and I had 2 of the subs next to each other... all 3 within about 4 feet of each other and no problems occurred. Right now, my PM 5.1 sub is placed next to me in the left front corner of my room and my PM 4.1 sub is placed to the rear in the right rear corner of my 10.5ft X 11ft room (yup... my college apartment is damn small... all the better for my speaker set up).