I received several good comments in response to my inquiry about the hypothetical instance of invalid and valid consecrations by concelebrants at the same Mass. This response was too long for the combox, but is of such detail and is so worthwhile that I’m posting here with the permission of its author, who asked to remain anonymous. I think I can reveal that this contributor has done advanced studies in theological areas:
At the very least, I think we have to say that a Mass in which principal celebrant says the wrong words of consecration is of doubtful validity. The Holy See has never issued a responsium ad dubium to this question but I would tend to lean toward the idea that the Mass is not valid in the case you mention for the following reasons.
The concelebrants do not hold the species. The principal celebrant does. It’s his hands that are holding the species and connecting his words to the species in his hands.
First, we must remember that there is a distinction between the sacrament and the sacrifice of the Mass-—the sacrament is the Real Presence in the elements, the sacrifice the offering of Christ to the Father. Traditionally, the sacrifice is effected/consummated when the celebrant receives both species. Hence, a priest who consecrates both species but receives only one has validly confected the sacrament but has not consummated the sacrifice (and while this does not invalidate the sacrament itself, it does invalidate the Mass and the priest should not receive a stipend for it). The same is true if a priest consecrates one species but unintentionally not the other (e.g., speaking the words of consecration of the bread over both species). The properly consecrated species is validly confected, even if the other is not-—although now the sacrifice is not complete until the other species is confected and received by the priest. In the Middle Ages, there were questions of what to do when a priest dies in between consecrations. Universally, the answer was always that another priest should finish with the rest of Mass so that the sacrifice may be completed. The importance of this point is not to be underestimated. The Code of Canon Law Canon 927 says explicitly, “It is absolutely forbidden, in even extreme necessity, to consecrate one matter without the other or even both outside the Eucharistic celebration.” The Latin here for “absolutely forbidden” is actually nefas, which has no proper English translation but is an ancient word which is related to something like “against the divine will.” It’s used only six times in the Code, and this is one of them. Though the Canon does not mention the validity of such a consecration, the importance of the sacrifice is highlighted here.
The celebrant must hold the sacred species