Is it really crossing over into a MPD however? Voice dialogue is a therapeutic modality where you take a 2nd person perspective on your psyche components in addition to 1st/3rd person. There's something to it but it's not MPD. The most popular example is the "inner child" complex which people often reconnect with when they empathize with their own child.
While journaling I have a random personality picker which draws from a list of subjectively helpful archetypes. From favorite actors to inner child and future self, and more. Then I interview them or converse with them intuitively, with the aim of exploring the situation or problem from a new perspective.
Related is a theory that by taking the perspective of a given personality, we essentially become that personality (not person, but personality) for however short a time period, giving ourselves access to their tools for problem-solving...
Discussed in Stephen Buhner Plant Intelligence and the Imaginal Realm.
If you find this interesting I highly suggest looking at the "Big Mind" process that's been incorporated into western Zen Buddhism.