My best guess, based on my observations of the markets, is that most vendors who know how to rig clothes/hair in both Poser and DS will usually package the two versions together.  I think it depends on the amount of work involved and the vendors' preferences, but it seems to me that most will choose that option.  I'm also guessing that some vendors who only know how to work in one platform will take a leap of faith and try to learn at least what they need to know to convert to the other system, asking a lot of questions to be sure they do it right and getting a lot of helpful feedback and answers from the community.  There will, of course, be some that prefer to only support one platform, and that is okay. Everyone has different time constraints going on in their lives.  I think there will be an increase in tutorials around the topic of clothing conversion from one platform to the other as a result of this. 

In most cases, morphs will only need to be created once for a given clothing/hair.  They'll just need to be transfered by the various means described in this thread over to the other platform's figure version or loaded into each platform's figure version individually from the modeling program they were created in, but the morphs themselves (in most cases) will only need to be modeled/formed/created in whatever outside program they're being created in once.  It's more a case of the loading them into the figure needing to be done twice or converting them over from one platform's figure to the other, not so much a creating of the morphs themselves done twice.  Morphs to flip collars and skirts, I imagine, will be easily useable in both programs.  In most cases, it's just the setup of the morphs that needs to be done twice or converted over.  Hope my repetative babbling makes sense. 

I know some morphs are created right within DS or Poser.  Those would need one of the various conversion techniques discussed somewhere in this thread.  Most vendors, I've observed, prefer to make their morphs in an outside program, though.

I'm open to critiques or others clarifying what they know I actually mean.  This babbling seems clear to me, but one never knows.