First of all, to determine the keys to writing a vocal duet, we must first define what a vocal duet is. Here is an encumbercing definition;
Vocal Duet:
A vocal duet is a musical and lyrical composition made for two performers (vocalists) in which the performers (vocalists) have equal importance to the piece. It is a back and forth between the two voices (perspectives) in the song, wherein one sings, expressing their perspective, sentiments, feelings, and, truth, and the other correspondingly does the same, with each having lines within a verse or verses or their own verse that they sing. It features solos from each individual vocalist in sections and them, also singing in partnership - most often times in the chorus. Also, they may harmonize at points. It can be written for a male-female partnership, a female-female partnership, or a male-male partnership.
Now that we know what a vocal duet is and what it encumbrances, what are the keys to writing one. Now, what I think and most definitely feel makes a vocal duet is that back and forth, the symmetry between the two voices (vocalists). It's when those voices correspond to each other and how well they correspond, the symmetry they have in relating to one another, and how well that relation comes across to the listener and in the song. Now, although this symmetry is important, if the overall composition (the lyrics, melody, and music) is subpar, your vocal duet will be subpar, lacking, so, the first key would be the quality of its parts. You want well-written and constructed lyrics, for one. Also, you want a good melody, as well as good accompanying music. But I stress again that lyrical content is of the utmost importance.
Okay, we've stated that lyrical content is important, but why you might ask, because it is your feature. It is the focal point of a vocal duet. Everything else supports or serves to highlight that content. Through constructing the lyrical content and getting the feel of the flow thereof, you can establish a melody. Although you can, also, first, establish a melody and write your lyrical content around that. But whether it's content first or melody first, your song, words, need to have a rhythm, and the music accompanying it needs to follow that rhythm. Symmetry is good when it comes to a duet. You want symmetry between the lyrics and music and symmetry between the two voices (vocalists), wherein the voices and what they sing correspond.
Now that we're focusing on the overall symmetry of our composition. We can then examine what makes for good symmetry. As far as the lyrical content, the two voices within the song, it's the back and forth between the two. Any really good duet I've heard had and has a conversational style to it. That style is part of the key to capturing your audience and constructing a good or great duet. The conversational style duet makes the audience feel like they're a party to what's unfolding, to what the parties are feeling, and to that moment that they're having. As the voices (vocalists) go back and forth speaking to one another and stating their feelings, perspective, and truth, the audience feels let in on that and are allowed to feel the sentiment, feelings, emotions, of the song and of the individual voices therein. And if one and / or both of those voices express what they themselves (the listener) feels, they are drawn in further to the song. The rapport of the voices within the song and the relationship and correspondence of the content sung, expressed, is what makes a duet impactful.
So, let's summarize what we have; one: you want well-written lyrical content, two: you want it to have a good rhythm (flow), three: you want it to have a good melody, four: you want the accompanying music to rhythmically match was written and for it to be good, And most of all, you want all of that to blend well and to construct it in a conversational style to draw in the audience. If you can do all of these things and have a well-written and alluring chorus to bring the verses together and reinforce your theme (message), You have written a good duet, whether it be your classic romantic slow jam or a more upbeat duet.
