Setting up stereo Rel subwoofers….

I started with 1 REL sub, which I set up as perfectly as I could...when I added the second, placement was relatively easy, one of the benefits of dual subs...the key to placing the second was asymmetrical placement ,  guided by Duke L (AK Sub Swarm)...some depends too on bass of main speakers...mine are large floor standers with good bass...adding the second sub was even more beneficial than I had hoped...

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