I wait with FH grip which is between E and SW. I shift - rotate the racket a wee bit - to the conti righ hand and E/SW left hand for my 2 handed BH. I never spin racket to accomplish a grip change.
I keep both hands on the racket until I start my stroke. Basically, the 1st thing I do is try to recognize where my opponenets shot is going and make the shift to the appropriate grip keeping both hands on the racket during the shift. That 2 hands on for FH and BH
Some 2 handers wait to return with a FH grip on the dominate hand and also have the non-dominate hand in E or SW ready for 2 HBH. I don't do this - I wait with FH grip on dominate hand, and cradle the throat low - just above the handle. I only switch to BH grips when I recognize ball is going to BH.