This article seems relevant (if someone can find a less terrible pdf, I would appreciate it). Abstract:
The illusion of independent agency (IIS) occurs when a fictional character is experienced by the person who created it as having independent thoughts, words, and/or actions. Children often report this sort of independence in their descriptions of imaginary companions. This study investigated the extent to which adult writers experience IIA with the characters they create for their works of fiction. Fifty fiction writers were interviewed about the development of their characters and their memories for childhood imaginary companions. Ninety-two percent of the writers reported at least some experience of IIA. The writers who had published their work had more frequent and detailed reports of IIA, suggesting that the illusion could be related to expertise. As a group, the writers scored higher than population norms in empathy, dissociation, and memories for childhood imaginary companions.
The range of intensities reported by the writers seems to match up with the reports in r/Tulpas, so I think it's safe to say that it is the same phenomena, albeit achieved via slightly different means.
Some interesting parts from the paper regarding dissociative disorder:
The subjects completed the Dissociative Experiences Scale, which yields an overall score, as well as scores on three subscales:
The subjects scored an overall mean score of 18.52 (SD 16.07), whereas the general population score a mean of 7.8, and a group of schizophrenics scored 17.7. Scores of 30 are a commonly used cutoff for "normal" scores. Seven subjects exceeded this threshold. The mean scores for the subscales were:
The latter two subscales are considered particularly diagnostic of dissociative disorders, and the subjects did not differ from the population norms on these. They each had only one subject score over 30 (not the same subject).
What I draw from this: Tulpas are the same phenomenon as writers interacting with their characters. Creating tulpas doesn't cause other symptoms associated with dissociative disorders. There shouldn't be any harmful long-term effects (if there were, we should have noticed them in writers). That said, there are some interactions that some people have with their tulpas that are outside the range (to my knowledge) of what writers do:
The tulpa community generally endorses the first two as being safe, and claims the last to be horribly dangerous and reliably ending in insanity and/or death. I suspect the first one would be safe, but would not recommend trying any of them without more information.
(Note: This is not my field, and I have little experience with interpreting research results. Grains of salt, etc.)
Very few people have actually managed switching, from what I have read. I personally do not recommend it, but I am somewhat biased on that topic.
Merging is a term I've rarely heard. Perhaps it is favored by the more metaphysically minded? I've not heard good reports of this, and all I have heard of "merging" was a very few individuals well known to be internet trolls on 4chan.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.