My wife (just married this past May!) and I used this song.
Perfect because I like the Allison version, and she prefers the Keith version.
http://www.youtube.com/watch?v=E3mptQi9k-A
You can down-load the song from here and has never been available commercially. It was mixed by a DJ down in the states. Their voices mix quite nicely.
http://www.sendspace.com/file/ijfutw
Our last song of the night was Adam Sandler - I wanna Grow Old With You