Also,
http://en.wikipedia.org/wiki/Locked-in_syndrome What I was mentioning earlier.
As well as a permanently conjoined twin. For many reasons.
You'd never have privacy, you'd always be gawked at, you'd have to work with them well, since you share a body.
You'd never have sex alone.
You could never ignore them, and you'd never truly be alone. You'd be restricted when it comes to developing as a person, for instance, my conjoined twin may not enjoy looking at horrifying gore in an attempt to wtfpwn a thread on 4chan, which would cause problems.
Plus, if they die, it could possibly kill you too, and if it happened after years, that'd cause a lot of psychological damage. I'd probably, rather than kill myself, kill the twin and say it was a suicide. Get that thing cut off, and have a decent life knowing I was a sibling-killer.
Although, masturbating with two minds instead of one might be delightful, if you could coerce them into it.
http://www.youtube.com/watch?v=BkKWApOAG2gAlso, you can tell the one on the left of the photo was the one supposed to grow into that body. I'd be pretty pissed with the other one. .-.