Mimamsasutra 167.2

167.2

vyapadeśabhedaś ca bhavati, bahu dugdhīndrāya devebhyo havir iti, bahu dugdhi mahendrāya devebhyo havir iti, ato 'pi devatāntaram, ekadevatātve mantravikalpaḥ syāt.

And there is a difference in designation: “Milk abundantly an oblation for Indra, for the gods,” and “Milk abundantly an oblation for Mahendra, for the gods.” On this account too, it is a distinct deity. If there were only one deity, there would be an option between the mantras.