The Essence of the Twin Seas drops from a rare spawn Dune Thresher named Razormouth. He is also part of the Bounty: Kill Razormouth.
Take the Howling Plateau waypoint and follow the road north. Past the bridge is the Black Canyon Mines where you can find Razormouth usually beneath the sands. Look out for movement beneath the sand. The scales of his backbone can be seen. It drops in Normal difficulty too.