@sudssuz - in the rare occasions the siblings themselves discuss that period they always talk in the plural, yeah. But then given between the two of them Koma is almost certainly the stronger and definitely the more aggressive one even in their natural "pure" state it'd seem logical she'd be the altogether bigger menace when gone all Demonic Edgelord and hopped up on Dark Side bad voodoo.
Also given the general weirdness of the ayakashi on a good day it's entirely possible they were melded into one body or something similarly bizarre back then. Compare Kokkuri-san's regular Fusion Dance with Chika for ex, and she isn't even running on crazypants demon mojo. ¯\_(ツ)_/¯