So, hey, let's think about the sex lives of conjoined twins instead. A terrific piece from Alice Dreger:
One thing we know for sure about the sexuality of conjoined twins: People who aren't conjoined are fascinated by it. At least it seems that way, judging by the number of reporters calling me to ask about the sex lives of conjoined twins since the A&E reality show Abby and Brittany went on the air several weeks ago. As I've told callers, although there are no real studies of the sex lives of conjoined twins, we can safely assume that conjoined twins want—and occasionally feel conflicted about wanting—sex, as we all do....
So, I suppose I should get to what the people really want to know: what do conjoined twins feel when they have sex? If one is sexually stimulated, does the other feel it? If one has an orgasm, does the other enjoy the same, however unwittingly?
The short answer is that we don't know. Conjoined twins, like the rest of us, tend not to talk in great depth publicly about their most intimate moments. Based on what we know about the significant variability of one conjoined twin to feel a body part (e.g., an arm) that putatively "belongs" to the other twin, it's hard to guess how any conjoinment will turn out in practice. Nerves, muscles, hormones, and psychology all probably factor in to who feels what. If twins share one set of genitals, they're both going to feel any touching down there. Whether or not both are "having sex" with the third person in the equation depends on how you think about "having sex."
Go read the whole thing. (And if you're genuinely curious about the subject of conjoined twins, you might want to pick up Dreger's book, One of Us: Conjoined Twins and the Future of Normal.)