Transfer thru premium
Hey friend,
Apply H1 transfer thru a premium processing and
Join company B only after getting the approval in hand.
because if you start workin for B right now without getting H1 transfer approval, your company A will cancel your H1 and you will be totally dependent on B 's result. its best to stick to A until you get a approval from B unless of course B wants you to join in a very short time frame but still you can go for an early H1B premimum so that once you get it approved you can start with B.
hope this helps...
Valley guy