All perfect praise be to Allah, The Lord of the Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
Giving birth unnaturally by means of artificial insemination is permissible in case of necessity and provided that the conditions mentioned by the scholars are met. Accordingly, the egg must be taken from the wife and the sperm must be taken from the husband then the fertilized egg must be planted in the uterus of the wife, and all of this must take place while the couple are actually living together by virtue of a valid marriage contract.
Moreover, planting the wife`s egg in another woman`s uterus isn`t permissible no matter what the case may be. On the same lines, withdrawal of wife`s eggs while married to one man then getting them fertilized by another man after divorcing the first isn`t permissible as well since such an act leads to corruption.
In conclusion, each marriage contract should have its sanctity and privacy. Therefore, overlapping isn`t permissible. And Allah knows best.