I am seeing claims like this on several forums by different people

If you are closer or farther from one set of subs or the other, generally out of phase will sound better and produce a flatter response. If they are equidistant from your listening position, generally they will need to be in phase.

This sounds wrong to me. Their thinking is that by setting them out of phase, that they'll then blend together nicely by canceling each others peaks. But isn't the reason for having subs in multiple room locations to excite the rooms modes differently, not to comb filter each other?

Here's a perfect of example of something else I never tried - two subwoofers. But common sense can still prevail.

Unless one of the subs is in a really screwy place, it's obvious both should have the same polarity. (Not the same as phase.) However, the phase adjustments, if present, can also be tweaked. In that case the goal is to measure the LF response at high resolution, and adjust the phase knobs for maximum output around the crossover frequency. This applies to a single sub with a phase knob too.

--Ethan