From Swami Nikhilananda’s translation of the Upanishads. These two verses occur at both Svetasvatara 4.6-7 and Mundaka 3.1.1-2:
Two birds, united always and known by the same name, closely cling to the same tree. One of them eats the sweet fruit; the other looks on without eating.
Seated on the same tree, the jiva moans, bewildered by its impotence. But when it beholds the other, the Lord worshipped by all, and His glory, it becomes free from grief.
Nikhilananda’s commentary – based on that of Sankaracharya – is available here (pg 120 of the PDF)
From Chhandogya Upanishad (7.1-3) in Swami Nikhilananda’s translation:
Narada approached Sanatkumara [as a pupil] and said: “Venerable Sir, please teach me.”
Sanatkumara said to him: “Please tell me what you already know. Then I shall tell you what is beyond.”
Narada said: “Venerable Sir, I know the Rig-Veda, the Yajur-Veda, the Sama-Veda, the Atharva-Veda as the fourth, the epics and ancient lore as the fifth, the Veda of the Vedas (i.e. grammar), the rules of the sacrifices by which the Manes are gratified, the science of numbers, the science of portents, the science of time, logic, ethics, etymology, Brahma-vidya (i.e. the science of pronunciation, ceremonials, prosody, etc.), the science of elemental spirits, the science of weapons, astronomy, the science of serpents, and the fine arts. All that I know, Venerable Sir.
“But, venerable Sir, with all that I know world only; I do not know the Self. I have heard from men like you that he who knows the Self overcomes sorrow. I am on afflicted with sorrow. Do you, venerable Sir, help me to cross over the the other side of sorrow.”
Sanatkumara said to him: “Whatever you have read is only a name.”