I don't see why it wouldn't be possible to do doubleshot caps for the two-piece keys, but they would have to be pretty thin.
For the single piece keys, I think the way they are made is actually out of a single piece of plastic, so I don't think it would be possible to do them as double shots.
Also, having had both PBT dye-subbed keys and double shot keys, I have to say I prefer the dye subbed models. They just feel better under my fingers. Of course, that's only my preference, and I could see arguments the other way, as well.