The solution to this is easy if you know what to do with the ID's; in the Term inspector, you can find out the UWDAID of some concept A, and then call up a Synonym of some other concept B and change its UWDAID to that of A. That's all there is to it. Unfortunately, this is not intuitive and if an actual author wants to do this kind of thing, it would probably be better to provide a more self-explanatory mechanism.