Both are instrumental singular, but what‘s the reason for two forms? Is the second form just an older usage? I’ve seen this a lot for other words too.
Yes, older, more literary usage. I remember one native speaker (Sergei, if memory serves) saying in another thread that it is sometimes preferred inorder to make the distinction with other cases clearer, e.g. in songs.
As far as I can tell, all feminine forms, whether in nouns or adjectives, have this alternative singular instrumental ending in ою
I hear “с тобою” fairly frequently in songs. In addition to whatever feeling it adds from being older/more literary, it gives the author one more tool to make the rhymes work and to match the cadence of the words to the beat of the music.
Irrelevant addendum: I also hear too many syllables being squeezed into too few beats in Russian pop, beyond what I think would be considered acceptable in English-language songs. It’s probably something to do with subtle grammatical shifts being able to add several syllables onto the end of your word. ))
Thank you both for the clarification!