"If two dads in a family each desire biological parenthood, as is very often the case, we split the eggs from your chosen donor and fertilize half with the sperm from one dad and half with the sperm from the other. Assuming you are open to having twins, we then transfer the best embryo from each man and freeze all the remaining. About 50% of the time twins will result (one from each father) which of course share the same genetic mother. The other 50% of the time transferring two embryos results in one baby. Since all the other unused top-grade embryos at the time of your transfer will have been frozen, you can then have the option of coming back in the future (if a sibling is desired) and thawing and transferring embryos from the non-biological dad. The fact that by then your original donor is likely to be no longer available (since they tend to donate for a year or two and then move on) is not a problem, because the embryos already exist.
"Fortunately freezing and storing embryos for future use is safe and as effective as using fresh embryos, as long as they are of high quality (which is seldom a problem when using young egg donors).
"Sometimes the situation exists when biological fatherhood is much more of a priority to one man than to his partner, for whatever reason. In that instance you still might choose to create embryos from both dads but preferentially transfer only one man’s embryos until a pregnancy results, and then in the future (perhaps) draw from the supply from the other father which will have been stored for possible future use.
"When dual fertilization is chosen, even if some (or all) of the embryos from both men are eventually never transferred, each potential father needs to have undergone the full set of screening tests that are required by the FDA to permit fertilization to occur."