A couple that met at a gathering where the purpose was to meet with "like" individuals had the same physical condition, although mirrored, which was that the woman had one arm and leg missing and the man did as well, on the opposite side.

In a glimpse of a distant future, medical technology has advanced quite a bit and, even without anti-rejection medication, people are able to fuse with another, get transplants and swap body parts without any negative effects.

Andy and Amanda choose to merge in order to augment their existing abilities and leave behind those ineffective conditions.

These are not twins and not related, they're just a couple with a common goal and the ability to achieve it. Conjoined twins would not have differing physical genders.

While it's not shown here the model is capable of easily producing both gender associated genitalia at the same time, and on the "proper" side. The quotes are with understanding that some may chose to swap those sides and the model is versatile enough to allow it.

There's a lot of possibilities here. I've accidentally merged 3 people, I'm not sure if that's a good thing but it was interesting.