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Having both DID and PTSD, which is pretty common for people with DID, just means treatment takes longer and may have to change focus on a regular basis.
Therapy usually starts by building safe coping skills to decrease the damage PTSD-focused therapy can cause, and by trying to decrease the amount and frequency of dissociation.
Since many traumatic memories may be split up with different alters, you usually have to work on building cooperation and co-consciousness between alters. Then once that gets better you shift back into PTSD work to deal with the specific traumas and triggers. This is where treatment can feel like it lasts forever, and where you dance back and forth between exploring trauma and backing off to keep from making the dissociation worse.
It's also hard because some alters may not want to participate in therapy, or may be fearful of what other alters know, which can really slow or even bring therapy to a halt. As the PTSD and the dissociation begin to improve you usually see more integration or natural blending start happening within the DID system.
Unfortunately, sometimes this blending brings the PTSD issues back up because alters who didn't know some traumas are now having to cope with the trauma issues/images/damage that other alters brought with them into a merge or blend.
Some EMDR professionals work with DID clients and have special tools and techniques to help them deal with PTSD and DID. EMDR can be a great method for dealing with PTSD trauma that is resistant to talk-therapy (such as pre-verbal, or trauma that happened to alters who don't want to talk about it). But the professional must have special training and experience in dealing with DID patients to keep things from being overwhelming or harmful.
Medication, biofeedback, and mindfulness-cognitive therapy techniques can all be combined to further help the PTSD symptoms and help treatment continue without making the DID more complicated.
So the basics are the same, it's just more difficult, takes longer because you have to move slower to prevent worsening dissociative barriers, and takes longer because you have to repeatedly process many of the same traumas with different alters who may hold different parts of the same incident. We generally see that the earlier traumas occurred, the more frequent traumas occurred, the severity of dissociation/amnesic barriers within the system, and the larger the number of perpetrators involved all increase the difficulty in treating DID and PTSD.
Books on Amazon worth checking out:
Rebuilding Shattered Lives: The Responsible Treatment of Complex Post-Traumatic and Dissociative Disorders by James A. Chu
Trauma Model Therapy: A Treatment Approach for Trauma Dissociation and Complex Comorbidity by Colin A. Ross,