Interesting! There are already some great NiF daemon AUs (for example I love for every atom belonging to me as good belongs to you.) I think if LS/MCS' daemon didn't change form he'd figure out how to separate the way witches do and then come up with some way to pretend. But the most difficult part for MCS would probably be that his daemon being the same would force him to confront the fact that he's not a different person now, he's still (also) Lin Shu and the seeds of Mei Changsu were already in Lin Shu, so he can't separate the personalities as clearly as he tries to do in canon.