I am autistic and have DID. I do not have statistics on this or anything, but over the years I think I have only run into two other people with DID who were also on the spectrum.... found them on trauma based forum type places. I think there are a few people on this forum who have either DID or a dissociative disorder of some type. I don't remember who or which but it's in some what dx do you have type thread. I remember being surprised that I was not the only one here who has a dissociative disorder... you don't run into that all too often. I don't anyway. So yeah, I would say it is not common at all. Anxiety and depression.. oh yeah, common. DID not so much.
As for something along the lines of, does 'someone in my system' have autism but 'someone else' does not... I dunno. For me, no. 'We' all have autism. I have some in my system who might seem less autistic than I do or more so, but in my case it tends to boil down to how that version of me reacts to environmental stress. Stress sets me (damn near 'all' of me) off and amplifies my autie traits like nothing else does. Some versions of me cope better, others worse, so yeah... still autistic regardless. So yeah, it's a coping skills thing in my case. But again, just me. No idea about other people.
The whole thing gets kinda tricky though because things like child neglect/abuse/whatever can cause a child to grow up and present as possibly on the spectrum.. poor social skills, aversion to change, difficulties with eye contact, speech delays or difficulties, sensory processing issues, rocking as self soothing, and so on. Really, it does require a damn competent psych professional to help pick all that stuff apart.
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"...don't ask me why it's just the nature of my groove..."