For your goat mated on Dec 23rd you probably should stop milking her two months before she is due to give her time to build up her body again for a new lactation. But it is good to get her into a routine and used to you, then start to dry her off over a week's period at the end of March.
Well my kids and unmated milkers are still coming into season here, so I think your other girl may well be in-kid.