Ever read "When I Look in the Mirror I See Double" by etothepii on AO3? I'm stealing that idea. Vex and Vax are one person, one mind, one consciousness... two bodies. They've figured out how to live like two people -- Vex can do certain things Vax can't and vice versa, but effectively they just have twice the time in the day. It's normally easy enough to pretend to be two people, even when they're right next to themselves, since they have plenty of practice. Everything gets a little more difficult, though, when they fall in love with a certain angsty gunslinger.
+smut, of course, this is the kinkmeme
+the awkward explanation; Percy doesn't believe it, and they have to... demonstrate
+either they/them pronouns for Vex&Vax (they're literally a plural person!) or randomly switching between she and he when referring to the individual consciousness