When you buy one of the two skins from the store, it locks you out from getting the other one. I bought Dawn Soraka and it wouldn't let me buy the Night skin. I tested my hypothesis out by refunding Dawn Soraka and it let me buy Night Soraka, but locked me out of buying Dawn Soraka. The store doesn't let you buy the other Soraka skin if you already own one of them.
Edit: I didn't pay attention to the RP price tag before. The Soraka skin you don't own also doesn't receive the discounted RP price.
Edit 2: This bug is in game. Glow effects on Dawnbringer Soraka's wings become really bright. It looks like whenever she gets the passive movement speed towards allies who are below 40% health the glow gets brighter, and the glow only goes away when she dies.