[Betty and Casey] ... cannot both be identical with the original zygote for, given the transitivity of identity, that would imply that they are identical with each other, which they clearly are not. We must conclude, therefore, that when monozygotic twinning occurs, the zygote that divides thereby ceases to exist and two new zygotes begin to exist at that point .... What this means is that, even if most adult human organisms begin life at conception, monozygotic twin organisms began to exist somewhat later, when a zygote that began life at conception divided. (Jeff McMahan, The Ethics of Killing, pp. 25-26)Maybe I'm missing something, but it seems most natural to say that there are two entities here, Betty and Casey, with an overlapping stage--Zygote (A). A is a stage of both Betty and Casey. Betty isn't identical to A, and Casey isn't identical to A, because A is just a stage. Nevertheless, A is the first stage in the life of both Betty and Casey. They start their lives when A begins.
It seems like lots of things have overlapping stages like this, or at least you can dream up lots of things. Take for example The Two Funerals. Two funerals take place during the same two-hour time period in a funeral home. The scheduler messed up, and the reception for each takes place in the same room. Mourners talk to each other about their respective deceased, sign the same guest book, drink from the same wine bottles, etc. Then the two funerals split apart, with services taking place in two separate rooms. Paralleling McMahan's view of the twins, you'd have to say three funerals took place--the reception-funeral, which ended at the end of the reception, and then the two services that took place in the separate rooms. But no, surely, there are two funerals, which shared a first stage.
Boy, what fun I could have dreaming up more overlapping events like this. Two parades start off separate, but merge for 10 minutes, then separate again. Using McMahan's reasoning, we've got to think there are actually five parades here. At the merge, the first two come to an end. Then there's the merged parade. After the merge, another two come into existence. Five, count 'em, five!
If you think life generally begins on the first day of gestation, it seems to me you can think so for identical twins as well. But is that what we should think? I would like very much to think about personal identity and gestation without playing abortion-chess--i.e. looking ahead to the implications of various views for abortion ethics. Most of the time when we're coming to terms with when life begins, we're thinking about a life that continued (our own, our children's lives) or that will continue (a wanted pregnancy). It seems like this ought to be the context for reflection at least some of the time, instead of abortion ethics always being the elephant in the room.