Up above, Jim Dixon mentioned "TWO SLEEPY PEOPLE," by Hoagy Carmichael and Frank Loesser. Loesser did a lot of conversation songs with one person singing over the other - "BABY IT'S COLD OUTSIDE" was mentioned above, but not tied to Loesser. His ultimate conversation song was "FUGUE FOR TINHORNS" from Guys and Dolls (I got the horse right here).