I'm not a geneticist, but I read a couple of those registered cases and at the present this situation is only possible as the result of (a) genetic defect(s).These sets of twins are monozygotic, which we generally call "identical twins", but thanks to a chromosomal abnormality they're not identical.
Of course, in Ive's day this may very well be treatable - or even optional, I wouldn't want to tell the author what his world is like - but unless the doctors reverse the gender of one of them, at the best you could call them "monozygotic with discordant sex twins", which doesn't sound nearly as good as identical...
PS don't take too long studying this, get to writing that next chapter!