Board Thread:Skyrim/@comment-12323583-20131106174950/@comment-4057437-20131118065556

138.9.75.207 wrote: You can't be a werewolf and a vampire at the same time.

You can be one, max out its tree and then switch. It is easier if the final form is done after the less desired one, but it is not impossible the other way. (Getting lycanthropy a second time is harder than getting made into a vampire again.)