For a number of years, while at Lancing concelebrating each morning with my colleagues (except on those mornings when I said a Latin Mass in a different Chapel with the boys and masters who liked that sort of thing), I was disconcerted by a colleague who had a habit of edging his voice ahead of mine even when I was Principal Celebrant. Why should he, I fretted, snatch the Mass from me by consecrating ahead of me and leaving me without the substances of bread and wine to consecraate myself? So I developed a habit of getting a good head of breath earlier in the Institution Narrative so that I could keep in sync with him. I needn't have bothered. A Pope sorted this problem out for me 800 years ago.
Innocent III (1198-1216; was he the pope under whom England became a feudal subject of the Holy See?) takes it for granted that "from time to time many priests concelebrate" and adds "the Cardinal Presbyters of Rome have been accustomed to stand around the the Pontiff and to consecrate together with him" - a pretty blunt and authoritative indication from the Bishop of Rome as to the meaning of the Rites of his own Church. What concerns him is this very question of what happens if they don't keep their voices together at the words of Consecration. "Is the one who first pronounces the words the only one who confects the Sacrament?" His answer to this is that "Whether the priests utter them before or after, their intention must be referred to the instant at which the Bishop says them, with whom principally they are concelebrating, and then all consecrate and confect at the same time".
I wonder if S Thomas Aquinas agrees with that? I'd better have a look in the Summa.
This series continues in the near future.