Conjoined Twins

So I met these conjoined twins...

Basically, it was just one woman with two heads, but the "other" one had a totally separate personality. Anyway, I got to know them a bit and it turned out they hated each other. I asked why, and the head in the middle said the head on her shoulder had deliberately fused with her in the womb. I asked her about it and she agreed. She said she just wanted to quit while she was ahead.