6 But here He proceeds: “While I was with them, I kept them in Your name.” Since I am coming, He says, to You, keep them in Your name, in which I myself have kept them while I was with them. In the Father's name, the Son as man kept His disciples, when placed side by side with them in human presence; but the Father also, in the name of the Son, kept those whom He heard and answered when praying in the name of the Son. For to them had it also been said by the Son Himself: “Verily, verily, I say unto you, whatsoever you shall ask the Father in my name, He will give it you.” But we are not to take this in any such carnal way, as that the Father and Son keep us in turn, with an alternation in the guardianship of both in guarding us, as if one succeeded when the other departed; for we are guarded all at once by the Father, and Son, and Holy Spirit, who is the one true and blessed God.
But Scripture does not exalt us save by descending to us: as the Word, by becoming flesh, came down to lift us up, and fell not so as to remain Himself in the depths. If we have known Him who thus descends, let us rise with Him who lifts us up; and let us understand, when He speaks thus, that He is marking a distinction in the persons, without making any separation of the natures. While, therefore, the Son in bodily presence was keeping His disciples, the Father was not waiting the Son's departure in order to succeed to the guardianship, but both were keeping them by Their spiritual power; and when the Son withdrew from them His bodily presence, He retained along with the Father the spiritual guardianship.
For when the Son also as man assumed the office of their guardian, He did not withdraw them from the Father's guardianship; and when the Father gave them to the guardianship of the Son, in the very giving He acted not apart from Him to whom He gave them, but gave them to the Son as man, yet not apart from that same Son Himself as God.
Source: Tractates on the Gospel of John (New Advent)