The sacrifice of the Mass is, first and foremost, Christ offering His atoning sacrifice through the ministerial priest. Objectively, therefore, two Masses are always greater in power than one, because the saving act of Christ, the main offerer, is once again applied to sinners—at least to those who are disposed to receive the effect prayed for.
Subjectively, the reverence of the one offering can obtain a greater or lesser reception of said graces for himself and for others, and to such an extent that the piety with which a Mass is offered by, for example, a Saint Philip Neri, may lead to the reception of more graces than many Masses offered by a less reverent priest. But although that is true, the overall state of the universe is better, one can say, due to the objective fact of many representations of the sacrifice of Calvary, than it would have been were there fewer. No amount of subjective devotion, which has to do with the reception of the fruits of the Mass, can ever equal the objective propitiatory and impetratory worth of a single Mass offered by Christ the High Priest through His minister.
When two priests concelebrate one Mass, a single act of sacrifice is made present through both of them together, acting in tandem as one instrument—as when several men pull on a rope together, there is one pulling of the rope, and one effect, e.g., that a heavy stone be pulled. In contrast, when two priests celebrate two Masses, Christ makes present anew, through each of them, His sacrifice to the Father; for us men and for our salvation, He has twice renewed His oblation at their consecrated hands. This multiplies the graces poured forth into the world from the Lord's most holy soul, the fountain of all gifts.
It is quite true that the concelebrants may have diverse Mass intentions and receive separate stipends, but it is no less true that among them there is no other consecration than the one that takes place on the altar at which they are all simultaneously present. Hence, there are not numerically many representations of the sacrifice of Calvary, and so there cannot be the same multiplication of the effects inherent to the offering of Mass. With separate Masses, the offering of Christ’s saving Passion is multiplied—in a sacramental way, yes, but one that is nevertheless real, not merely symbolic. As St. Thomas writes: “The oblation of the sacrifice is multiplied in several Masses, and therefore the effect of the sacrifice and of the sacrament is multiplied” (Summa theologiae III, q. 79, a. 7, ad 3).
In the encyclical Mediator Dei, Venerable Pius XII teaches that the essence of the Sacrifice of the Mass—that is, the sacramental representation of the Lord’s Passion—consists in the separate consecration of the two species of bread and wine, which makes present in a mystical way the death of Christ, when His body was separated from His blood. In a concelebrated Mass, there is only one consecration (one might say: una consecratio, multi consecrantes). Therefore, as compared with a series of private Masses, there are fewer representations of the one Sacrifice of the Cross and fewer applications of its saving power to the living and the dead.
According to the Thomistic school, of which Fr. Garrigou-Lagrange may be taken as representative, Christ the Eternal High Priest from Heaven actually offers each Mass offered through His priestly instruments on earth. The question with regard to concelebration would be, then: Does our Lord offer one hundred times when one hundred priests concelebrate, or only once? He offers only once, using the hundred concelebrants simultaneously as His instruments in effecting a single Transubstantiation and therefore a single enacting of the Sacrifice. It is like the difference between starting a single fire with one hundred matches or starting a hundred fires with as many matches: in the former case, those many matches function, essentially, as a single fire-starter, but in the latter case, they function as many fire-starters. If the Lord’s purpose in coming to earth was to set fire to it, as He Himself declared (cf. Lk 12:49), would we not want His ministers to be igniting many distinct fires every day? There is, after all, no limit to the need of the human race for this warming, illuminating, and transformative fire.
There is, of course, a canonical limit, based upon reasons of fittingness, to the number of Masses a priest may celebrate on a given day, and there can be legitimate reasons for not offering Mass on a particular day (e.g., sickness, lengthy travel, a time of war or persecution). Apart from such circumstances, there is no compelling objective reason why each priest should not daily exercise the most sublime action of his own priestly power, his ontological share in the eternal high priesthood of Christ, to give perfect worship to God and advance the salvation of the world.
Appealing to the earlier distinction between objective merit and subjective disposition, one might attempt to neutralize this conclusion by stating that a devout concelebrant will obtain more graces from the Mass than a distracted or rushed celebrant will do from a private Mass. Although this is true, in theory, of the fruits of the Mass for the individual, it does not touch the earlier argument about the objective value of the renewal of Christ’s sacrifice, the moreso as it is more often offered. More to the point, given how much easier it is to be focused on the sacrificial action and prayers in individual celebration, the subjective disposition of two priests concelebrating is very likely not going to be equal to that of the same priests celebrating individually, so the question seems an academic one. A priest friend once joked that he could prove this claim with a video camera: the disposition of concelebrants rarely seems as intent and concentrated as that of a single celebrant. Thus, in practice, concelebration will almost always confer less grace upon its many ministers and win less grace for the people than individual celebration will confer upon its single agent and the people for whom he is offering it.
Ceteris paribus, it is better for the priest himself, for the People of God, and for the world outside the Church that each priest should celebrate Mass daily, if nothing beyond his control prevents him from doing so. In this way, he most fully actualizes the potential of his ministerial priesthood, letting the Lord Jesus Christ, Eternal High Priest, make the maximum use of him as the instrument by which the fruits of His saving sacrifice will be poured out upon the human race from the rising of the sun even to its setting.