2 DD12's vs. 1 DD18

Any thoughts on which combination will give the most output? How about the best sound? This is in a HT system only, run off a LFE output from the HT prepro, in a room approx. 13' x 15'.
As an owner of the DD12 I would say go with 2 of the 12's, my room gets by fine with one but my ambition is to eventually sell the DD12 and get 2)DD10's. Every system I have heard with 2 subs sound markedly better and more controlled than with one.
I had tried both ways exactly what you are doing and similar room size as well. Two DD 12 is much better and that's what I have now.
I would go with the 12s. I have heard one in my room which is 21x29 and it was plenty. I now have a single 15 but a single 12 would have been enough.
Go with a single DD12. That is more than enough for your room. If you still feel like that's not enough, go for the DD15, that's a fantastic subwoofer. I personally don't believe having two subwoofers is a good idea both acoustically and financially.