Objection 1. It seems that the form of the consecration of the bread does not accomplish its effect until the form for the consecration of the wine be completed. For, as Christ's body begins to be in this sacrament by the consecration of the bread, so does His blood come to be there by the consecration of the wine. If, then, the words for consecrating the bread were to produce their effect before the consecration of the wine, it would follow that Christ's body would be present in this sacrament without the blood, which is improper.
Objection 2. Further, one sacrament has one completion: hence although there be three immersions in Baptism, yet the first immersion does not produce its effect until the third be completed. But all this sacrament is one, as stated above (Question 73, Article 2). Therefore the words whereby the bread is consecrated do not bring about their effect without the sacramental words whereby the wine is consecrated.
Objection 3. Further, there are several words in the form for consecrating the bread, the first of which do not secure their effect until the last be uttered, as stated above (4, ad 3). Therefore, for the same reason, neither do the words for the consecration of Christ's body produce their effect, until the words for consecrating Christ's blood are spoken.
On the contrary, Directly the words are uttered for consecrating the bread, the consecrated host is shown to the people to be adored, which would not be done if Christ's body were not there, for that would be an act of idolatry. Therefore the consecrating words of the bread produce their effect before. the words are spoken for consecrating the wine.
I answer that, Some of the earlier doctors said that these two forms, namely, for consecrating the bread and the wine, await each other's action, so that the first does not produce its effect until the second be uttered.
But this cannot stand, because, as stated above (5, ad 3), for the truth of this phrase, "This is My body," wherein the verb is in the present tense, it is required for the thing signified to be present simultaneously in time with the signification of the expression used; otherwise, if the thing signified had to be awaited for afterwards, a verb of the future tense would be employed, and not one of the present tense, so that we should not say, "This is My body," but "This will be My body." But the signification of this speech is complete directly those words are spoken. And therefore the thing signified must be present instantaneously, and such is the effect of this sacrament; otherwise it would not be a true speech. Moreover, this opinion is against the rite of the Church, which forthwith adores the body of Christ after the words are uttered.
Hence it must be said that the first form does not await the second in its action, but has its effect on the instant.
Reply to Objection 1. It is on this account that they who maintained the above opinion seem to have erred. Hence it must be understood that directly the consecration of the bread is complete, the body of Christ is indeed present by the power of the sacrament, and the blood by real concomitance; but afterwards by the consecration of the wine, conversely, the blood of Christ is there by the power of the sacrament, and the body by real concomitance, so that the entire Christ is under either species, as stated above (Question 76, Article 2).
Reply to Objection 2. This sacrament is one in perfection, as stated above (73, 2), namely, inasmuch as it is made up of two things, that is, of food and drink, each of which of itself has its own perfection; but the three immersions of Baptism are ordained to one simple effect, and therefore there is no resemblance.
Reply to Objection 3. The various words in the form for consecrating the bread constitute the truth of one speech, but the words of the different forms do not, and consequently there is no parallel.
Source: Summa Theologica (New Advent)