Setting up stereo Rel subwoofers….

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.

@jastralfu

If he sets it up the way REL states like I posted above, the subs won’t need to be moved, the only thing he may have to do is adjust the subs volume down slightly.

REL will have you moving the sub around during set up if you only have a single unit, to take advantage of boundary reinforcement because of the lesser output.

@newfzx7 i agree, that’s why I was questioning setting up a single sub then adding the second.  Do both at the same time as REL, and you, recommend.  I did end of moving my REL subs arrangement though into an asymmetrical placement that works better for my room.