Here’s a quick liturgy question. If two priests are concelebrating Mass and the principal concelebrant messes with the words of consecration to such an extent as to render the Mass invalid normally, but the co-celebrant uses the correct formulation, is the Mass still invalid? And what does the person in the pew do if he’s sure of what the principal celebrant said, but not sure what the co-celebrant said?
This is theoretical, but relevant I think.
The concelebrant “covers” the validity for the whole Mass. It’s not like the concelebrant consecrates some of the hosts and part of the wine, but both consecrate all species reasonably present. However, there remains the danger of scandalizing or worrying the faithful. If that were to happen to me as a concelebrant, and someone asked after Mass, I’d tell them “I got you covered, don’t worry.”
Normally, you wouldn’t hear what the concelebrant says, anyway, during the words of consecration, since he’s supposed to say it in a low voice, and he’s probably doing it from memory, anyway.
I guess the answer to whether the concelebrant validly ALSO validly consecrated or not is matter of judgment, but remember, the concelebrant isn’t making stuff up as he goes along, or using his personal script, probably, so it’s most likely valid (unless you know Fr. Concelebrant goes off and makes serious changes).
Speaking of which, I’ve been a concelebrant in Masses where the celebrant goes off on tangents (thankfully not invalid stuff, but extraneous fluff), which makes it really hard for us concelebrating. I tend to think to myself in this case, “Hey bro, mind letting us know when you’re coming back to Earth, so we know when our speaking part is coming up?”
I’ve seen something close to this in practice.
An elderly priest became distracted (at the ellipsis), saying “This … is the blood of the new covenant”, etc. I could hear the concelebrant (a recent ordainee) clearly pronounce the words. It passed without comment.
Incidentally, the same thing happened with the priest about 4 months later, with no concelebrant. The server (and a pew-person) intervened.
As to your question, theologically I think that it is clear that the Mass is valid: Christ and His Priests are One Priest (Council of Trent), and it is enough that one concelebrant who possess the priestly character to have proper form and intent.
Psychologically, at least in the example I report, it is debatable whether it is better for the concelebrant to ask the principal celebrant to repeat. See, for instance, the avoidance of scandal/wonderment that Pius V desires in #10 of De Defectibus.