Jaiminīya Upaniṣad Brāhmaṇa 2.6.2

2.6.2

sa yadi brūyād dvau ma āgāyeti prāṇa udgītha ity eva vidvān dvau manasā dhyāyet
dvau hi prāṇāpānau
dvau haivāsyājāyete /

If he should say: “Chant for me for two”, knowing that breath is the udgītha, he should mentally focus on two.
For out-breath and in-breath are two.
Two indeed are born to him.