Has a general term been coined to describe a fantasy being that inhabits two separate bodies (which are not physically attached) but is, in fact, one single being?
Not as far as I know. You'll probably have to make something up. But if you're doing steampunk or more modern you could probably try dopleganger - assuming the two bodies look the same.
Alternatively assuming its older epic fantasy and you want a historical feel not to mention some sort of divine background, you could go for some of the twin gods - Castor and Pollux spring to mind.