According to Michael Knudsen from PopBytes, Katy Perry will release another 'promo' single thing from 'Prism' before the actual second single.
"OMG! I left out #DarkHorse, yet another hot track from @KatyPerry which will be available on 9.17 — then 9.30 you'll have #WalkingOnAir!" he tweeted having been to a listening party for the album.
So basically 'Dark Horse' will be out on September 17 on iTunes, then 'Walking On Air' two weeks later and then, we guess, the actual second single after that.
Bit confusing but there we are.