As indicated by the topic title, I have yet to ever see paired enemy units move, similarly to bosses. Even more interestingly, once I kill the lead unit and their partner is left on the title alone, that unit won't move either.
Is this just a weird AI thing, or a gameplay balance/difficulty thing? Do they move in later chapters, or perhaps on Lunatic? I'm on Hard Mode, Chapter 8, for what that's worth.