Before a taddhita affix having an indicatory ñ , ṇ or k , the vṛddhi is substituted for the first vowels of both members of dvanda samāsa of the names of devatā-s,
As अग्निमारुती in अग्निमारुतीमनड्वाहीमालभेत् ॥ आग्निमारुतं कर्म ॥ The rule applies to Dvandas relating to hymns (सूक्त) and sacrificial offerings (हवि). Therefore, not here, स्कान्दविशाखौ देवतेऽस्य = स्कान्दविशाखः 4.2.24. So also ब्राह्मप्रजापत्यम् by ण्य from ब्रह्मप्रजापती ॥ See 6.3.26.
The short इ in the आग्नि in आग्निमारुत, आग्निवारुणम् is by 6.3.28. A compound relates to a sukta, which worships a deva through hymns; and that by which a 'havis' is determined, is a compound relating to sacrificial offering.,