A contributing factor could be the time of the mass and the rehearsal. Several years ago a survey indicated that TIME of day is the overriding factor in their decision as to which mass they attend. If this helps, I'm glad; but I'm not trying to incite a flaming war!

Is it maybe nothing to do with either of you, and maybe about them discovering they prefer that form of the Mass? Roborgel and Matthew both make good points, especially about preference for Mass times (and it's def a trend with servers, who are often dependent on what Mass their parents go to). If that's the case...

That’s the breaks basically.

That being said,...

We have 4 weekend Masses at our NO parish, and each is differently "flavoured", musically. There are Masses I prefer over others because of the music. I am involved in two of them on the regular, and I know which one I would pick if I had to choose only one. Time of day is a factor, but the music has always been the bigger influence.

Have you asked them why? Maybe it's rep. Maybe it's the ordo. Maybe they like how you run rehearsals better. And then probably have a chat with your counterpart.

It could depend on the NO repertoire. Is it vertical, that is God centered with robust Eucharist hymns or very horizontal, anthropocentric, with weak or ambiguous Eucharistic realism?

Many singers who are serious about sacred music reach a threshold where they crave text and music with stronger Eucharistic realism. If the repertoire gap between the two Masses is wide, that shift in musical diet is often the primary driver, rather than Mass timing or rehearsal styles.

Regarding the pastor: I wouldn't drag Father into this unless an actual conflict arises. Involving clergy over routine volunteer movement often turns a non-issue into official 'drama.' A quick, warm, heads-up to your fellow director ('Hey, just letting you know [Singer] reached out to me—wanted to keep you in the loop so there's no weirdness') handles the optics, preserves your working relationship, and keeps Father out of the weeds.

@Don9of11, you said exactly what I wanted to say but couldn't find the words. Thanks.

Thanks for the kind words, @Abbysmum!

The reason this topic is so top-of-mind for me is that I’ve been examining it through my fifty years of choral service across two parishes—mostly at St. Mary's in Akron, Ohio—and through a local hymnological study of our parish inheritance: You can download the study in its entirety here. A Parish Inheritance

Looking through the historical repertoire of a single parish over the decades really drives home what @Liam was talking about regarding the shift in musical diets. To be clear, I’m neither a champion of chant nor a proponent of the TLM—my passion is protecting the treasure of hymnody and exploring "catechesis through melody." Sacred hymns have a lineage in Christian worship that predates the formal codification of Gregorian chant, serving as a primary tool for teaching and instilling the faith.

Tracing how the texts and hymn tunes changed over time shows a direct correlation between what a choir is fed musically and how their liturgical appetite evolves. When singers are exposed to strong Eucharistic realism, robust hymnody, and vertical, God-centered texts, their standards naturally shift—and once that threshold is crossed, it's very hard to go back to a lighter "diet."

When singers are exposed to strong Eucharistic realism, robust hymnody, and vertical, God-centered texts, their standards naturally shift—and once that threshold is crossed, it's very hard to go back to a lighter "diet."

It’s not just singers. I gave up eating meat one Lent so I could afford the gas to drive four hours round trip to go to Mass (sometimes would do it the Saturday evening prior as well so I could go to Confession as it wasn’t particularly available in the city I was living in) after being exposed to Latin Gregorian chant. It wasn’t even high quality chant, but it was far better than the insipid horizontal music* sung at the church I had available to me at the time. I was able to make arrangements to continue my education off campus, so I ended up just moving there to save money and less oil changes.

* Ashes on Ash Wednesday was the song that made me make the definitive decision, but the stuff they sang made the St. Louis Jesuits sound like traditional music. At least their music is scriptural.

Since you have both forms at your parish, I can see it as an opportunity for a bit of "cross-pollination." Perhaps invite the NO choir to sing with the TLM on a Requiem on All Souls' Day and promote it as an opportunity to sing it in its intended context and not just a concert or bits and pieces at NO Masses, and then see if your counterpart will invite the TLM choir for a special NO Mass like a parish feast day or the bishop's visit or whatever. Take it as an opportunity to work together as the last thing you want to have happen is the phenomenon of "parallel parishes." And certainly ask the "defecting" NO choir members what made them come over to the TLM choir in a non-confrontational way; as others have said, it may, and likely does, have nothing to do with the other guy.

As a bit of an aside, my previous parish offered both forms as well, and they now have 2 seminarians, a man in formation for the permanent diaconate, a woman religious novice, and a woman looking into consecrated virginity. Some of them were cultivated in the NO, others in the TLM. You have a great opportunity in your position.

While that is a potential pitfall I would presume Caleferink, who mentions this as a recommendation - and the TLM choir in discussion - would already be quite cognizant of which Requiem compositions to choose for the sacred liturgy.

Most of the most popular and commonly sung Requiems were not written to be sung in a liturgical setting.

OH?
Which ones specifically are you thinking of? I mean, besides non-requiem requiems like Brahms, Delius, Britten etc. I really don't know of any settings using the liturgical text that were not intended for the liturgy.

The problem is not of intent; it's in the way that the Requiem has changed over the years. For some early Requiems (Ockeghem, Certon) some movements have different texts, in which case you replace them with chant or another setting. The rubrics are a bigger issue; the Church has gotten (needlessly) restrictive over the past century or so. If you accept De Musica Sacra et Sacra Liturgia as the last word for the Missal of Pius V, you really can't do an accompanied Requiem, even just with organ. You could sneak it by if the organ is just doubling the voices. But forget about the Mozart (or even Durufle!). It's worse in the Missal of Paul VI because of the changed texts and the elimination of the Sequence (which is a BIG chunk of the Mozart).

Law enters when Taste leaves. In Cleveland, they did the Mozart in a TLM for Pope Benedict, and it was very moving, but I found myself thinking that Pius X had a point. And nobody was a Karen about the rubrics, because it was for the Pope. And orchestral Requiems are not "commonly sung" in the liturgy, for the obvious reason. I think that singing a polyphonic Requiem together on All Souls is a great idea. And it doesn't have to be difficult. The Zucchini Missa pro Defunctis is very easy, the Casciolini not much harder. With the Renaissance ones you get the scoring problems you get in all Renaissance polyphony, but if you can work around that, some are pretty simple, Nor would one have to do the whole thing (and probably could not, if liturgical).

OH?
Which ones specifically are you thinking of? I mean, besides non-requiem requiems like Brahms, Delius, Britten etc. I really don't know of any settings using the liturgical text that were not intended for the liturgy.

-Verdi’s Requiem
-Faure’s Requiem
-Gossec’s Requiem
-Berlioz’s Requiem
-Dvořák’s Requiem

Heck yeah. I love that. I’m actually going to take a page from Doug O’Neill’s playbook and announce our choir when sending my announcement to other choral directors and the choral professors here (I need one more pro, I need volunteers, and we need subs/ringers). And I might make mention of this sort of thing.