Śatapatha Brāhmaṇa ("of a hundred paths") 4.4.4.3

4.4.4.3

atha yasmātsamiṣṭayajūṃṣi nāma | yā vā etena yajñena devatā hvayati yābhya eṣa yajñastāyate sarvā vai tattāḥ samiṣṭā bhavanti tadyattāsu sarvāsu samiṣṭāsvathaitāni juhoti tasmātsamiṣṭayajūṃṣi nāma

And as to why they are called Samishtayagus. Whatever deities he invites at this sacrifice, and for whatever deities this sacrifice is performed, they all are thereby 'sacrificed-to together' (sam-ishta); and because, after all those (deities) have been 'sacrificed-to together,' he now offers those (libations), therefore they are called Samishtayagus.