My reaction to this is it would take three very special people. It might work if this were a true polyamorous triangle or quad. You said something about both doms having their own sub. I think the child and the adults would need to have all the adults involved in parenting decisions. Just as a couple who have a child may have different ideas of child rearing and different roles in rearing the child, I think the sub would also need to be involved in rearing the child in a similar identified or agreed upon role. The baby and later child needs love and security, with consistency and stability being part of that security. It does not know that having three or four adults who love and guide it is not "normal" at least not until it gets to school.

I would wonder about bonding though. Is the sub going to nurse the child? Would you try to induce lactation in yourself? Would you bottle feed? How would the sub feel about you nursing the child while she was drying up? Nursing is clearly what is best for the child and the mother who has delivered the child.

If you later broke up, what if any visitation rights would the sub have. That is an awful lot of emotional energy to invest in a child to later have no rights. I think the roles and the ifs would have to be very clearly decided beforehand, with the expectation that after the child arrives emotions may differ from what was predicted. The child is not a possession. It's well being would have to be the basis of all decision making.

Due to personality differences etc. A child will often bond more strongly with one adult than the other. what would happen if the child's strongest bond was with the sub and you later separated. Could you handle only having visitation rights, because that is what is emotionally best for the child? Even though your DNA will be a part of the child, the subs hormones and biochemistry will strongly influence its development while in her womb. One might then argue that biologically the child is equally a part of her.

This is a very complex issue and would have to be well thought out and legally spelled out before hand. If our society was more commonly polygamous, it might not be such a big issue, as such contingencies would have already have become custom, but you would be breaking really new ground here.