Setting up stereo Rel subwoofers….

If you have room, REL recommends putting each subwoofer just outside of each main speaker.  If that doesn't work, set them just inside of the mains.  I emailed them and asked them when I was setting mine up. 

For 2 subs, REL recommends placing the subs to the outside (or inside if there's not enough room) of each main speaker. Also, slightly behind the front of the speaker.

Hook them up as you said above. Follow REL's instructions for making adjustments to the crossover and volume on one sub at a time (disconnect or turn off the other sub).

That's how it's done according to the manual for my pair of T5x and it worked perfectly. Not sure if the procedure is different for other models.

Connect Rel stereo subs with one to Right and one to left channel. You will need to tie the red & yellow together for the + and the black to the -.. If there is a ground hum you may need to float the ground(s). If you can't adjust the sub volume past 1/2 way you can untie one of the + (Red or Yellow). You can find this info on Rels website

Connect it to one channel only.  I don’t think there is any reason not the connect both subs following the placement suggestions from REL though and then moving subs if needed.  The original placement of the single sub might not be optimal once you add the second anyway.

First, are you using high level inputs?  If so easy to connect the R to the Right amp inputs and the Left to the Left amp inputs using high level wiring, which I recommend.  As said, this is all on the REL website.  Different amps (class D) require different wiring.  For my class A/B Audionet amps, I use the Red and Black to the inputs on the amps and float the yellow wire.  For the crawl testing of only one sub (maybe this is what you are asking) I don't think it matters much and I would just connect it to one side as this will be the final wiring.