Presumably, the OP is referring to two singers who don't normally record together doing so as a one-off venture (single or album), which therefore prohibits countless wonderful tracks by regular duos (most notably Simon and Garfunkel, the Everly Brothers and the Righteous Brothers) from being included.
Here are some great tracks, which fall into the category of what I perceive the OP requires:-
Something Stupid - Frank and Nancy Sinatra
Some Velvet Morning - Nancy Sinatra and Lee Hazelwood
Passing Strangers - Billy Eckstine and Sarah Vaughan
Mockingbird - Carly Simon and James Taylor