You cannot drink from the second without drinking from the first.
Drink from the Old Testament to slake your thirst, and from the New to quench it completely.
Compunction is found in the Old Testament; joy in the New.
Notice how the Lord, on his servants’ behalf, countered the wiles of the devil.
With deceitful cunning the devil beguiled one man in order to overthrow all mankind in his person.
But with salutary food Jesus redeemed all mankind, in order to restore with him all, even him who had been beguiled.
The Lord Jesus poured out water from the rock and everyone drank. Those who drank from the symbol were satisfied, but those who drank from the reality were inebriated.
That was a good inebriation that steadied the walk of the sober mind; that was a good inebriation that watered the gift of eternal life.
Drink of this cup, then, of which the Prophet said: Your cup that inebriates, how noble it is!
Drink the cup of the Old Testament and of the New, for in both you drink Christ.
Drink Christ because he is the vine; drink Christ because he is the rock that poured out water.
Drink Christ because he is the fountain of life; drink Christ because he is the river whose running waters give joy to the city of God, and because he is peace, and because out of his heart will flow rivers of living water.
Drink Christ to drink the blood which redeemed you; drink Christ to drink his words: the Old Testament is his word; the New Testament is his word.
Holy Scripture is drunk and swallowed when the power of the eternal Word penetrates the depths of the mind and the virtue of the soul.
In short, we do not live by bread alone, but by every word of God. Drink this word, but according to its own order.
Drink it first in the Old Testament; then hasten to drink it also in the New.
Ambrose of Milan (c. 337-397): On Psalm 1, 33 (CSEL 64:28-30); from the Monastic Office of Vigils, Saturday of the Sixth Week in Ordinary Time Year 1.