Single vs. Dual Subs

The technical theory behind multiple subs stacks up and there’s a lot of support for it. The only caveat I can see is that to make the most of it you need to be pretty flexible about where those subs go, if they just go right behind the L/R speakers then it may be an imperceptible improvement. Also it’s worth remembering that your room is a 3d space so different height placements should be considered as well. Basically symmetry is most pleasing to the eye but probably the worst way to lay out your two subs. If you have the freedom to put them wherever you want then there’s a good chance you’ll hear the benefits.
You’ll also want to do a bit of research into how to systematically set them up either by ear or using a mic.

xcool - Our ears aren't good at locating low frequencies so subwoofers don't contribute to the stereo image. In fact you will probably find that the best locations are not near your speakers or symmetrical.
What you're looking for is for each sub to excite different room modes so that they balance one another out to some degree. If you're not familiar it might be worth looking up 'room modes' and 'standing waves'. There are also some neat tricks for finding good locations although the whole process is a lot easier with a measurement mic and some software. You need to also make sure that the speakers are 'time aligned' which you can do by measuring and with some simple maths.

Just for the record, I do this regularly with a single sub, traps and careful use of an EQ.

I guess the advantage to doing it this way is that there is less trial and error involved. If we ignore the EQ for a minute then room treatment only attenuates reflections but it happens over a wide range of frequencies. The only limit is how much space you're willing to give up to absorbers e.g. to treat a problem at 80Hz you're looking for 1m depth of treatment... 1.5m at 60Hz etc. With EQ I think the only compromise is that you are getting a flatter response at the listening position but probably making other places in the room worse.

Theoretically with multiple subs you should be able to cover multiple frequencies but it could easily turn into a game of 'whack a mole' where you treat one node only to reinforce another one. I guess some of you EQ each sub individually to get around this.

Personally low frequency room response is somewhere I'm willing to compromise... treat the worst modes and live with the rest. Each room has imperfections which are part of its character if you're willing to think of it that way.