Since all being 8 inch driver, you may choose any of the 2 subs..
When you are connecting the subs with an AVR, you can connect to the input labelled "LFE"..
You may also connect to both ports (white RCA & Red RCA female ports) from a single sub pre-out using single RCA to dual RCA cable (below picture).. With this method, there is gain in Sound Pressure Level (SPL), which is said to be the same as turning the volume knob on the AVR..
Since you are anyways considering dual, you may connect to single sub input labelled "LFE"..