It's overdone, but "As Long as You're Mine" from Wicked might be a good fit and you both are probably familiar with it. Another option may be "Nothing is too Wonderful to Be True" from Dirty Rotten Scoundrels.
Good luck!
"A bird doesn't sing because it has an answer, it sings because it has a song." -- Maya Angelou
How about "All or Nothin'" from Oklahoma? That's one of my favorites "Barcelona" from Company "Believe" from The Scarlet Pimpernel (so pretty!) "Come What May" from Moulin Rouge "The Riddle" from The Scarlet Pimpernel might work, even though it's supposed to be a trio...
Sorry, I tried my best to match the vocal parts...