But if it can happen at all, it seems problematic. why not just have one partner be inseminated at a time, as in real life?
well, not necesarily. many species have many-numbered offspring. it's a good thing, since it allows for a greater pool of kids such that if one or two dies the while effort put into getting pregnant and stuff isn't wasted, not to mention the fact that the genome is still perpetuated, which is always a good thing for genomes. Both pregnant at once also has several advantages: first, a girl could cum inside herself or something to self-impregnate: this might be associated with strong cultural taboos but in certain cases, such as catastrophes, it might facilitate reproduction. Also, co-pregnancy will should augment genetic and, perhaps more importantly, epigenetic drift. two brothers born from different wombs will be more different than two brothers born of the same womb, since it implies different genome modulation, different gene silencing, and different histone modulation. This augmented genetic and epigenetic drift makes for a more diverse poor to select from, so it makes evolutionary sense, since genetic diversity is an awesome thing to have just to survive disease, not to mention evolution. Lastly, it distributed the load on both parents, in such a way that for four children each subject only carries two of them. Large litters are problematic, not only they imply a greater organic load, but it makes each individual offspring less viable: in rabbits, for example, some of the fetuses, the ones that develop at the furthest end of the uterus tend to get squeezed between the roof of the cavity and their siblings, and so tend to be smaller, weaker,and more likely dead... by dividing the litter in two uterus, besides, each parent remains more functional, so they can go on with their lives, and also, less likely to die. human mothers bearing twins are in more risk that those bearing singletons both pre and post-natally, and I imagine the same happens with triplets versus twins.
so yeah, I don't think its a disadvantageous trait... and even if it is, hell, we have loads of disadvantageous traits... our respiratory system is an invagination, for Darwin's sake, that's retarded!