11. The two that have entered into the cavity of the heart are indeed the individual self and the Supreme Self, because it is so seen.
The Katha Upanishad speaks of two who have entered the cavity of the heart and enjoy the results of action in the body.
The question arises whether these two are the individual soul and the intellect, or the individual soul and Brahman.
The sutra establishes that they are the individual soul and Brahman, because numerals naturally denote things of the same class. Since one is clearly a conscious self, the other must also be conscious and therefore cannot be the insentient intellect.
The mention of both as enjoyers is explained as a collective way of speaking. In truth only the individual soul experiences karma, but the attribute is extended to the pair as a whole.
12. The subsequent passages specify qualities that clearly distinguish the two as the individual soul and Brahman.
The soul is described as the charioteer, while the end of the journey is said to be the supreme state of Vishnu.
This establishes one as the seeker and the other as the goal attained.
Earlier also the sage is said to transcend joy and sorrow upon realising the effulgent One seated in the heart, which confirms Brahman as the second of the two.