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.
Really? I had the impression that switching was relatively common among people who had their tulpas for a while. But then, I have drawn this impression from a lot of browsing of r/Tulpa, and only a glance at tulpa.info, so there may be some selection bias there.
I heard about merging here. On the other hand, this commenter seems to think the danger comes from weird expectations about personal continuity.
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.