I seriously doubt that. The DD15 and DD18 are still great subs despite their age. I have a pair of DD15s which absolutely crush my JL F110s in terms of bandwidth, SPL, etc. This is no knock on JL but a fact of the laws of physics.
The user interface on the DD series subs is excellent, allowing you to digitally adjust phase, slope, EQ, etc. I haven't found another sub to date with so much adjustability which comes in quite handy when integrating multiple dissimilar subs in a theater room.
The real question is, should you trade 2 small subs for 1 larger sub. For that answer you need to read this:
http://www.audioholics.com/tweaks/get-good-bass/two-subs
Now if the question was:
2 DD18s or 2 F112s, I'd personally chose the 2 DD18s if you have the space to accommodate them.