When gay couples use surrogacy to become parents, they may decide to have one child each in a single pregnancy. To do this, the surrogate must agree to the transfer of two embryos, since a multiple pregnancy is desired and this type of pregnancy carries a greater risk of complications.
If the surrogate consents to the transfer of two embryos, half of the donor's eggs would be fertilized with the sperm of one of the future parents and the other half with the other. Finally, the best quality embryos would be chosen from each parent to be transferred to the surrogate in an attempt to achieve a twin pregnancy.
There is a possibility that only one embryo will implant in the surrogate's uterus, so there would be a single pregnancy. Another option that may occur is that no embryos are implanted and pregnancy is not achieved. In the latter case, a new transfer would be necessary if frozen embryos remain.