i think its for two reasons (although i may be wrong): rhett has a slightly deeper voice and that Link predominantly sings the harmony in their songs. Maybe because Rhett sings the melody, which is what you would sing if it were a solo, its sort of what your ear follows. just some ideas, but i don't really notice it that much.
that is true! I thought that when i first bought their songs and was listening to them. Rhett either seems to be the louder one or the only one that sings. or the first to sing.