Rel Subwoofer phase question

start each on the same setting

then sit back and listen, change phase on one, does the bass impact seem stronger?  if so leave it

try the other one then, same thing, same question, pick the phase setting that is louder and more impactful

Probably not. If they are it kind of indicates that your left and right sides could be wired 180 degrees out of phase and at the very least your two subs are 180 degrees out of phase.

It's pretty challenging to phase align speakers without the aid of a RTA. Even subwoofer frequencies are a very short fraction of a second long.

The phase setting of each sub is dependent on their location in the room relative to your listening position.  So depending on where they are located, they might be both set the same, or they can be set differently.  In my last large listening room, mine were set differently (both my large REL's and my Martin Logan Descents) as verified by REW.

Since REL's only have a 0/180* setting, sometimes either setting might not be quite right and when using REW, you can see a hole or a peak somewhere in the frequency sweeps.  But, you can try just moving the sub that's causing these issues just a foot or two in the room and fix those issues, even while leaving the phase setting where it was in the *better* setting, (not necessarily the perfect setting before physically moving the sub.)