We first became aware that we had at least one alter, a syskid, while we were being treated in a partial hospitalization program (PHP) when we were 22, after I had attempted suicide and nearly died. I was in a coma for 3 days and they told people (like my therapist at the time, and my parents) that I wasn't gonna make it but 3 rounds of hemodialysis later and I woke up, miraculously with no discernible damage to my kidneys. And shortly after that I was enrolled in PHP.
At the time our syskid alter, who made a lot of trouble in PHP, would buy things for us and I'd come around to finding objects I didn't buy around our apartment. It got me to thinking that maybe the out-of-control behavior and mood swings I was experiencing wasn't just my bipolar disorder (or my autism/ADHD)... I struggled for a long time to name this alter, and he's changed names several times. We slowly began to uncover other alters, and we're still in the process of doing this.
My treatment team is focused mostly on my bipolar + autism + ADHD. I've brought up DID with my therapist and psychiatrist, but my therapist is very focused on "here and now" problems and some alters rarely front during therapy sessions; as for my psychiatrist, he seemed interested but since there's no medication for DID, not a lot he can do. I was in a DBT program in California and when I moved back to New York they helped me find my current therapist, who is DBT aligned (intensively trained) and also has extensive experience with autistic clients.
My psychiatrist in California was so amazing, we're still in contact though less than when I lived there, at one point I was sending her up to 3 emails per day because I'm very rapid cycling and a lot can happen in a few hours. I told her about my young alter who fronted in PHP and she suggested that it could be a severe manifestation of BPD. I can't say that she's wrong, it seems pretty common to have both and I think I probably meet criteria for both so maybe there is a link. But I didn't really bring the issue up again. Another thing, I've actually been hospitalized about 20 times and have very distinct opinions about certain hospitals being good or bad. Even the best hospital (UCLA) wasn't really prepared to deal with DID; they specialize in psychosis and mood disorders. DID is rarely addressed in inpatient treatment.