All I can say is, wow, if you don't listen to the music set forth in this thread, you are missing out. Great stuff posted by all! I would offer something that is not 2 different singers, but what I believe is the consummate blend of 2 different genres, done to perfection:
Kenny Loggins and Stevie Nicks - Whenever I Call You Friend
I'm not sure if this is 2 different genres. I'm not too familiar with Loggins other than he sang "Footloose". I've always thought Loggins as soft rock and Nicks as rock.