I was under the impression that the various units which assist in healing (medical and veterinary), education (storytellers/bards/etc.), and law enforcement were made unable to merge/split because it caused problems with the calculation of their special attributes. I have noticed in my current game (on SVN version 11430) that cartwrights have the capability to merge, and, when I checked the Civilopedia, there is nothing in their entries to indicate any of the mechanical "healers" are restricted from merging or splitting.