You would just "Y" it out with quality sub type RCA cables.... Speakers are tough for me, when I install, I always use the same exact speaker in all locations. My next install will be 6.1 with 6 JBL E30's as long as the owner agrees to their formfactor and looks.
Both nice AVR's.... Go with the one you feel has the most power. As far as cost -vs- power output -vs- pre/pro/video capabilities go, I still like the Yamaha 5890 AVR. I would consider both of your listed AVR's a small step up from the Yamaha 5890 though they may not have quite as much muscle...