The rear channels on pioneers cannot be bridged.
Turning on rear speaker subwoofer in the menu simply applies a low pass filter and a mono signal to the rear speakers. They are still 4 ohm stable as a pair, so you could run two 4ohm subs at 22w each, or one dual 4ohm sub at 44w total.
They ARE 2 ohm stable if you only use one channel. Using two, its power supply will probably run out of current and something will fail. Thats why they say to use only one channel at 2 ohms.
Not bridgeable by any means.