I would like to throw a couple of other ideas out there as well. These are just duets that I love listening to.
What Do the Simple Folk Do?, Camelot
If I Loved You, Carousel
Tonight, West Side Story
Say It Somehow, The Light in the Piazza
Move On, Sunday in the Park with George
The Riddle Song, Floyd Collins
Honeysuckle Rose, Ain't Misbehavin'
What You Don't Know About Women, City of Angels
Agony, Into the Woods
If Momma Was Married, Gypsy
You Were Dead You Know, Candide
Who Could Be Blue/Little White House, cut from Follies
Last Chance Blues, Jelly's Last Jam
Freedom, The Unauthorized Autobiography of Sam Brown
Come Up to My Place, On the Town
There Is No Other Way, Pacific Overtures
The Picture Show, Parade
The Wheels of a Dream, Ragtime
What a Piece of Work is Man, Hair
(I got a little carried away)
I also add my support and surprise that the initial list didn't include
A Little Priest
Namaste