Śatapatha Brāhmaṇa ("of a hundred paths") 2.3.1.23

2.3.1.23

sā yā pūrvāhutiḥ | sāgnihotrasya devatā tasmāttasyai juhotyatha yottarā sviṣṭakṛdbhājanameva sā tasmāttāmuttarārdhe juhotyeṣā hi dik sviṣṭakṛtastanmithunāyaivaiṣā dvitīyāhutirhūyate dvandvaṃ hi mithunam prajananam

What first libation (pûrvâhuti) is made, that is the deity of the Agnihotra, and to that (deity) it is accordingly offered; and what second one (uttarâhuti) is made, that indeed is equivalent to the Svishtakrit (Agni, the maker of good offering); whence he offers it on the north part (of the fire), since that is the region of the Svishtakrit. Moreover, this second libation is made in order to effect a pairing, for a couple forms a productive pair.