Two-score (4)
(Other definitions for duet that I've seen before include "Music for two" , "Paired performance" , "Work for a couple" , "Piece for brace" , "A song sung by two" .)
'two-score' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for forty that I've seen before include "-- winks, a short sleep" , "number of penitential days" , "Kip" , "Two score" , "Two-score" .)