I really doubt that tulpas have much to do with DID, or with anything dangerous for that matter. Based on my admittedly anecdotal experience, a milder version of having them is at least somewhat common among writers and role-players, who say that they're able to talk to the fictional characters they've created. The people in question seem... well, as sane as you get when talking about strongly creative people. An even milder version, where the character you're writing or role-playing just takes a life of their own and acts in a completely unanticipated manner, but one that's consistent with their personality, is even more common, and I've personally experienced it many times. Once the character is well-formed enough, it just feels "wrong" to make them act in some particular manner that goes against their personality, and if you force them to do it anyway you'll feel bad and guilty afterwards.

I would presume that tulpas are nothing but our normal person-emulation circuitry acting somewhat more strongly than usual. You know those situations where you can guess what your friend would say in response to some comment, or when you feel guilty about doing something that somebody important to you would disapprove of? Same principle, quite probably.