Futanari.... A subject I haven't tackled in my writing career...
I dunno if I even have that kind of.... oh wait, I think I have some of these in my encyclopedia... I swear.... How the fck?
A-anyways, there seems to be multiple routes due to futanaris having multiple body stuctures but the most common would be that they are beautiful looking people who are bisexual.
If you want them to share a body, the first thing they'll tackle is how they will likely share one body before talking about their own sexuality. They would be repulsed in having sex with a certain gender but slowly come in terms with one another and understand each other's strengths and weaknesses later on. I think the most probably dominant sex that would win though would be the male side as male hormones are more likely to dominate in a futanari. Unless the souls are either a fujoshi or into males.
I think there are more to talk about this but as I am not experience more into this topic and only have a general view on what futanari is, I'll likely not be good at writing something a concept I don't know so I dunno.