The Night and Its Moon by Piper CJ

Farleigh is just an orphanage. At least, that's what the church would have the people believe, but beautiful orphans Nox and fae-touched Amaris know better. They are commodities for sale, available for purchase by the highest bidder. So when the madame of a notorious brothel in a far-off city offers a king's ransom to purchase Amaris, Nox ends up taking her place — while Amaris is drawn away to the mountains, home of mysterious assassins. Even as they take up new lives and identities, Nox and Amaris never forget one thing: they will stop at nothing to reunite. But the threat of war looms overhead, and the two are inevitably swept into a conflict between human and fae, magic and mundane. With strange new alliances, untested powers, and a bond that neither time nor distance could possibly break, the fate of the realms lies in the hands of two orphans — and the love they hold for each other.