I was running dual HGS12IIs and then upgraded one of them to a DD-15, not so much because I wanted more spl, but because I had a nasty resonance issue. Shortly there after, I upgraded the other to a DD-12, so I can speak with some experience here.
I'm not speaking for Velodyne, but I've spoken with them in the past. I would definitely go with a single 15 before going to dual 12's, it's cheaper and will yield about the same performance, you won't have as many placement issues and no cancellation issues. For the cost of dual DD-12s, you could swing a DD-18 and giggle yourself silly for the next year or so. Don't buy into the hype some people push about a 12" driver being faster than a 15" driver, at least not in the case of the DD series. The DD-15 is every bit as fast and tight as the DD-12 and it hits harder and lower. I'm very anxious for an official review to come out on the new DD series, I have a feeling it will become known as the new reference subwoofer.
Optimally, I would have gone for a single DD-18, but size is an issue, and I also have a one year trade up policy at my local store that is a one for one deal, so I traded up one HGS12 for the DD-15 and one for the DD-12. Let us know what you decide and how it shakes out as well as your impressions on the DD series.