Śatapatha Brāhmaṇa ("of a hundred paths") 1.2.2.5

1.2.2.5

tadvā etat | ubhayaṃ saha kriyate tadyadetadubhayaṃ saha kriyate 'rdho ha vā eṣa ātmano yajñasya yadājyamardho yadiha havirbhavati sa yaścāsāvardho ya u cāyamardhastā ubhāvagniṃ gamayāveti tasmādvā etadubhayaṃ saha kriyata evamu haiṣa ātmā yajñasya saṃdhīyate

These two acts are done simultaneously. The reason why these two acts are done simultaneously is that one half of the body of the sacrifice no doubt is that butter, and the other half is this rice-offering. 'That half and this half, these two let us now take to the fire!' thus (they think): for this reason those two acts are done simultaneously, and thus this body of the sacrifice is joined together.