While switching to doubles is no big deal, I would want a lot of dives in them so they feel as natural as single tank diving. I don't think a cave class is the best time to be drastically changing your gear configuration. Think about someone who's been diving a "tech" jacket like a zeagle with singles and shows up to his full cave class with a new BP/W and a set of 130's. That with the content of the cave class itself seems like too much new stuff to deal with over the typical 4 days of apprentice and full. As soon as I knew I was going full cave, I got a set of doubles, had an expeirenced doubles diver give me some pointers and got in over 50 OW dives on them and about 15 intro dives on them. In a month I take my full cave class and I'm glad I don't have a completly new rig to get comfortable with while I'm 1200' back in some cave with my instructor watching my every move.