You only need one RCA 99.999% of the time. Very few recordings do not have the low frequencies combined on both channels. Just make sure you use the variable preamp outs. If you use the fixed outputs, you'll only do that once, can get expensive.
(1) check the powered subwoofer manual for input options and also your receiver manual
(2) Assuming you are not considering any direct LFE output from the receiver using a single subwoofer RCA termination cable into the powered subwoofer’s designated LFE RCA input:
- then just connect the receiver preamp RCA outputs to the RCA inputs on your powered amp subwoofer.using a longer run pair of decent RCA interconnects.