I.3.58, and for this reason, if for no other (though idea is older), the etymology is probably false! The epithet is also Iranian.
Twinned and especially paired gods are characteristic of the Rig Veda.

Thus Yama and Yam[=i] are twins; and of pairs Indra-Agni, Indra-V[=a]yu, besides the older Mitra-Varuna, Heaven-Earth, are common.] [Footnote 115: Perhaps to be omitted.] [Footnote 116: _Pischel_, Ved.

St.I.p.

48.

As swift-going gods they are called 'Indra-like.'] [Footnote 117: VIII.