I was wondering the same thing, so I ordered some D2's to compare.
+1 to everything LeCig said.
I took some pics too.
The performance seems pretty similar, the only difference I can tell is the thread and the D2 seems slightly slimmer and my ones don't have an o-ring inside, but the mouthpiece cap is a thicker silicone plug, whereas the D1 is thin, hard plastic and the filler seems to come furthur up the tube with the D2... but I havent done a test to see if it will hold more.
All in all they seem pretty much the same, but different.
I would have been better off just getting more D1 cartos and batts, instead of getting both models, but at least now I know.
(in the pics, the green one is the D2)