Go to CPDL and enter the search term "bicinia", which means (roughly) "music for two voices." You'll find many duets and collections of duets.

The term "Bicinia" (pl. of "bicinium") mentioned by @CGM is usually used for unaccompanied two part music. Most of this repertory thus stems from the Renaissance. For use at mass, there are two possible types of music:

• Free bicinia based on Latin bible verses like those by Lassus or de Castro. These are only usable for performance.
• Cantus Firmus bicinia based on chorales like those by Othmayr (Bicinia sacra, 1547) or my bicinia over psalm tunes. Many of the chorale melodies have been recycled in traditional and modern catholic hymn books with revised or new text underlays. You can thus use the bicinia alternatim with the congregation singing different stanzas (or even in a different language).

Since the baroque period, "Bicinia" in the above sense have become rare, because accompaniment on chordal instruments like the organ became ubiquituous. The term "Duo" typically refers to music for two voices with accompaniment and there is an abundance of repertory, albeit mostly for equal voices. An example for the combination baritone & soprano from the Baroque period is "Ave regina caelorum" by Rovetta, for a more romantic style my setting of "O salutaris hostia", and for a contemporary style my two part mass setting.

I adapt a lot of SATB music for 2 parts, usually by blending the alto and tenor lines into a new harmonic line. I am a mezzo-soprano and often sing with a tenor. We often flip back and forth between who is singing the melody line and who is singing the harmonic line.

@Abbysmum if I may revive this to follow up ... I would expect simplistically to keep the Sop, give Bass to Alto, and give up the Tenor. Are there examples and suggestions for how such adaptations can be done to maintain interest? Perhaps it's down to musical experience and taste to re-shape any given piece.

Also, Perosi's Laudate Nomen Domini (same conditions as above)

My edition: https://psallitedomino.com/store-1/p/jamoi215thpc6jv7p9cuxowm8c360z

Practice tracks: https://www.youtube.com/playlist?list=PLk71GctqoFYzjr6D4Z_yP9pRyisPvoJrk

Thank you @Xopheros. That's in mensural notation.
I since remembered that this is dealt with in Hawley Ades Choral Arranging, chapter 5 Two Part Writing. https://archive.org/details/choralarranging0000ades_e5u4
It's a challenge for small choirs a cappella to sound good in two parts without accompaniment to fill it in.

@Abbysmum if I may revive this to follow up ... I would expect simplistically to keep the Sop, give Bass to Alto, and give up the Tenor. Are there examples and suggestions for how such adaptations can be done to maintain interest? Perhaps it's down to musical experience and taste to re-shape any given piece.

@Xospheros The soprano and bass lines both have a tendency to come home to "rest" on the Doh, whereas the alto and tenor lines often rest at Mi or Sol, at least in the SATB arrangements I'm working with. So I'm often striving to maintain that relationship when arranging. I find alto lines as written can often have a lot of parallel 4ths (which make me crazy - my piano/theory teacher would have threatened me with her swords if I tried that!). Bass lines often have terrible voice leading, and I don't want to sing them LOL, so I strive to keep even the harmony line more melodic in nature.

Assuming everyone is on the same clef (and not the treble with the 8va for tenor), sometimes the harmonic line is below (like an alto) and sometimes above (like a descant), depending on the piece and the character I'm striving for. Even though I'm mezzo, I am fairly comfortable in my upper range beyond even the A5, so sometimes I'm just singing the straight alto or tenor line 8va from as written if I'm especially lazy that week (being careful not to overpower the tenor). I usually save those for when it's a recessional or something else robust that the congregation will sing heartily to, so I float on top of the melody.

This past week I just rewrote the entire harmonic line from scratch for a standard LM hymn because the accompanist was playing some other harmonic structure (because she didn't like the one provided in the hymnal). It only took me about 30 min start to finish, including engraving. I just had her text me a screenshot of her page with notes so I knew the structure she was going to play.