Originally Posted by Bob Mariani
Use a fernco coupling that is designed for this transition.
I agree with bob with the fernco but some states do not allow them and you have to use a stainless steel no hub transition band designed especially for going from cast iron to pvc. I don't care for them especialy underground but I know in memphis we had to use them so check with your locals codes.
as far as to which drain to tie into I would tie into the one that is closest to the tub. less concrete busting the better but always a good idea to check on venting and pipe size (2 inch min.)