How does the Catholic Church explain the juxtaposition of two tenets of faith with two different pre-conditions at Roman 10:9?

It makes complete sense to me.

To believe something whole-hearted we normally say we hold it within our 'heart', rather than our mind.

But men being mortal creatures cannot read the hearts of others, but can hear what they say. Hence to 'confess [this] with your lips'; that is to say.

I don't see that the Catholic Church has anything to explain here.

And nor do I see what is the problem you see here - except if you notice that certain Catholics or other evangelicals, particularly their leadership, their deeds don't match the words they are mouthing. Which might lead you to suspect bad faith amongst them.

How does the Catholic Church explain the juxtaposition of two tenets of faith with two different pre-conditions at Roman 10:9?

9 If you declare with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved. - Romans 10:9 (Douay-Rheims 1899 American Edition (DRA))

It is obvious one must be ready to profess what one believes in order to be saved.

The Apostle explains yet more clearly what is required in order to have part in the salvation of Christ. Not only is it necessary to believe, but thou must also confess with thy mouth, i.e., make public confession that Jesus is Lord (the literal order) of the universe, and therefore truly God. This means a public confession of Christ’s Divinity, such as was required before Baptism (Acts 8:37; Acts 16:31). Further, besides believing and confessing the Incarnation of the Son of God, it is necessary to believe in His Resurrection from the dead. Paul mentions these two mysteries because they are the principal ones of Christianity, those on which all others depend. If he speaks first of external, and then of internal faith, it is only because he is following the order of Moses’ words, which speak of the mouth first, and secondly of the heart. - Father Callan’s Commentary on Romans 10:9-18

St. John Chrysostomn writes the following:

Ver. 6, 7, 8, 9. “But the righteousness which is of faith speaketh on this wise, Say not in thine heart, Who shall ascend into heaven (that is, to bring Christ down from above): or, Who shall descend into the deep? (that is, to bring up Christ again from the dead.) But what saith it? The word is nigh thee, even in thy mouth, and in thy heart, that is, the word of faith which we preach. That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised Him from the dead, thou shalt be saved.

To prevent the Jews then from saying, How came they who had not found the lesser righteousness to find the greater? he gives a reason there was no answering, that this way was easier than that. For that requires the fulfilment of all things (for when thou doest all, then thou shalt live); but the righteousness which is of faith doth not say this, but what?

“If thou confess with thy mouth the Lord Jesus, and believe in thy heart that God hath raised Him from the dead, thou shalt be saved.” Then again that we may not seem to be making it contemptible by showing it to be easy and cheap,1486 observe how he expands his account of it. For he does not come immediately to the words just given, but what does he say? “But the righteousness which is of faith saith on this wise; Say not in thine heart, Who shall go up into heaven? (that is, to bring Christ down); or, Who shall descend into the deep? (that is, to bring up Christ again from the dead.”) For as to the virtue manifested in works there is opposed a listlessness, which relaxeth our labors,1487 and it requireth a very wakeful soul not to yield to it: thus, when one is required to believe, there are reasonings which confuse and make havoc of the minds of most men, and it wants a soul of some vigor to shake them thoroughly off. And this is just why he brings the same before one. And as he did in Abraham’s case, so he does here also. For having there shown that he was justified by faith, lest he should seem to have gotten so great a crown by a mere chance, as if it were a thing of no account, to extol the nature of faith, he says, “Who against hope believed in hope, that he might become the father of many nations. And being not weak in faith, he considered his own body now dead, and the deadness of Sarah’s womb. He staggered not at the promise of God through unbelief; but was strong in faith, giving glory to God; and being fully persuaded that what He had promised He was able also to perform” (Rom. iv. 18–21): so he showed that there is need of vigor, and a lofty soul, that takes in things beyond expectation, and stumbles not at appearances. This then he does here also, and shows that it requires a wise mind, and a spirit heavenly (Gr. heaven-reaching) and great. And he does not say merely, “Say not,” but, “Say not in thine heart,” that is, do not so much as think of doubting and saying with thyself, And how can this be? You see that this is a chief characteristic of faith, to leave all the consequences1488 of this lower world, and so to seek for that which is above nature, and to cast out the feebleness of calculation, and so to accept everything from the Power of God. The Jews, however, did not merely assert this, but that it was not possible to be justified by faith. But himself turns even what had taken place to another account, that having shown the thing to be so great, that even after it had taken place it required faith, he might seem with good reason to bestow a crown on these: and he uses the words which are found in the Old Testament, being always at pains to keep quite clear of the charges of love of novelties, and of opposition to it. For this, which he here says of faith, Moses says to them of the commandment,1489 so showing that they had enjoyed at God’s hand a great benefit. For there is no need to say, he means, that one must go up to heaven, or cross a great sea, and then receive the commandments, but things so great and grand hath God made of easy access to us. And what meaneth the phrase, “The Word is nigh thee?” That is, It is easy. For in thy mind and in thy tongue is thy salvation. There is no long journey to go, no seas to sail over, no mountains to pass, to get saved. But if you be not minded to cross so much as the threshold, you may even while you sit at home be saved. For “in thy mouth and in thy heart” is the source of salvation. And then on another score also he makes the word of faith easy, and says, that “God raised Him from the dead.” For just reflect upon the worthiness of the Worker, and you will no longer see any difficulty in the thing. That He is Lord then, is plain from the resurrection. And this he said at the beginning even of the Epistle. “Which was declared to be the Son of God with power ... by the resurrection from the dead.” (Rom. i. 4.) But that the resurrection is easy too, has been shown even to those who are very unbelieving, from the might of the Worker of it. Since then the righteousness is greater, and light and easy to receive, is it not a sign of the utmost contentiousness to leave what is light and easy, and set about impossibilities? For they could not say that it was a thing they declined as burdensome. See then how he deprives them of all excuse. For what do they deserve to have said in their defence, who choose what is burdensome and impracticable, and pass by what is light, and able to save them, and to give them those things which the Law could not give? All this can come only from a contentious spirit, which is in a state of rebellion against God. For the Law is galling (ἐπαχθὴς), but grace is easy. The Law, though they dispute never so much, does not save; Grace yieldeth the righteousness resulting from itself, and that from the Law likewise. What plea then is to rescue them, since they are disposed to be contentious against this, but cling to that to no purpose whatever? Then, since he had made a strong assertion, he again confirms it from the Scripture. - Homily XVII