I had a similar question regarding what happens once the child is actually here. If they are your sub are they living with you, will they continue living with you?
Let's say, for argument's sake, that the very real obstacles Twisted Tails brings up can be put aside in terms of the screening process; how will you handle the very complex and intense relationship between the surrogate, the two parents and child? How do you manage the emotional issues that, IMO, would inevitably arise? Would you then release the sub? Would that be understood from the beginning? What might be the emotional fall out?
It might be relatively easy to look someone you love in the eyes and tell them introducing the new life into the world won't change anything because the only tangible love is between you and them. However, once that new life is here, there is the potential for new, tangible and intense attachments.
It's an interesting scenario, but real world messiness is the looming issue and I have trouble thinking of any good solutions for the issues. I would think it would actually be more feasible if it were a sub that was not in a relationship with either of the Dominants in question.