Vanellope Nope, this was never directly answered in the book. I've heard many theories, including them being twins who shared one threep for cost reasons, conjoined twins, or a single person who either prefers to switch between playing male and female, or who has Schizophrenia, Multiple Personality Disorder, or something of the sort.
Tyler The book never explicitly offered an explanation for this, no. One theory is that they are conjoined twins yet this still does not detail anything about the line, "We blew up the balloon." This is definitely one of the loose ends left by the book, either intentionally or unintentionally.
Nirkatze A quote from Chapter 4 of the sequel, Head On, shows that Chris is just as in the dark: "I was told there was a reason the twins shared their threep, and I had been promised early on it would be explained to me, but a year after moving into the house I shared with them and three other Hadens, the reason had yet to be revealed. Honestly at this point I sort of enjoyed speculating more than knowing the actual answer."