On Showdown, my Ferrothorn and an opponent's Talonflame both got knocked down in the same turn. Here's how. I settled up Stealth Rock before Talonflame was sent out. I sent out Ferrothorn. Talonflame lost 50% of its HP. Talonflame used Flare Blitz, OKHOing Ferrothorn. Here's the catch: Talonflame took damage from Iron Barbs, Rocky Helmet, and Recoil damage, causing it to faint. Here is the link to the replay if you're interested.
https://replay.pokemonshowdown.com/gen7ubers-1146013014