In the liturgical tradition that many of us grew up in, there were, at Mass and in the Office, quite often 'commemorations'. After the collect of the day's main celebration, the collect [plus, in the Office, antiphon, versicle, response] of a lesser celebration was said. The same happened at Mass with the Secret and the Postcommunio. The 'reforms' associated with the aftermath of Vatican II enforced a rigid discipline of one celebration only on one day. Laurence Hemming in Worship as a Revelation, Chapter 11, reminds us that this had first been proposed at the Jamsenist Synod of Pistoia; he gives good reasons for revisiting this question.
I feel that the problem of 'NO commemorations' hits particularly hard on two days: S Valentine and S Swithun. Each of these days still has a standing in English popular culture; yet each is unobservable in the liturgy of those who adhere to the modern 'reforms' because there is, on the same day, a compulsory celebration on the Universal Calendar of another Saint or Saints. (Problems also arise with regard to SS Philip Neri, S Augustine, and S Bede; and with SS Hugh and Hilda.) The best one could do would be to mention the other Saint somehow in the Intercession.
Yesterday morning, saying a New Rite Mass of S Bonaventura (actually, I had done him the previous day in my EF Mass, but that's yet another problem), I suddenly, without premeditation, added a commem of S Swithun.This, like all such impulsive actions, caused a minor problem: the people were aleady sitting for the Reading. So, when I got to the Prayer over the Oblations, I resorted to another exstinct custom of yesteryear: I said the 'commemoration' under one conclusion with the collect of the day. Yes, I know that this is licentious individualism run riot.
The Orthodox do not have a problem with combining celebrations; I believe Old Calendarists still observe the complicated rules for combining Easter Day with the Annunciation. Our Tractarian forefathers rather enjoyed the opportunity to preach on our Lady of Sorrows when the Annunciation fell on Good Friday. Why should one only be allowed one theme on one day? What is wrong with the appetite for synchronic and diachronic fellowship involved in 'commemorating' a Saint whose main celebration was in an earlier century or is now in another place?