One of them eats the sweet fruit; the other looks on without eating

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)