that said, i completely agree with jaya ... 👏
clarity is the keyword here ...
put simply, given a normal singing range, the female voice is best suited to the lower and middle scales whereas the male voice to the middle and higher scales
this is cos the male voice is naturally at a lower scale compared to the female voice
for a duet song, one would normally choose a pitch at which both singers are comfortable so for a singer with a good range whether male or female it shud be easy enuff ... however sometimes it may so happen that either singer is uncomfortable with the range and so u have almost an inaudible or grating male voice at lower notes (with a sweet souding female voice at the same note) and shrieky females at higher notes (with a clear resounding male voice at the same note)
but it also depends on the singer and the mood of the song
hope this explains ... 😳
am trying to think of examples ...