A double shifty is a spin - usually a 360 with two shifts packed inside it. You throw a shifty the way you're already spinning, then bring it back the opposite way before you land. It's a reversal in the air, the unwind happens while you're still rotating.
How to do it
Hit the jump exactly like you would for the spin on its own - nothing changes on the approach, and your body shouldn't move any differently until you're already in the air. Once off the lip, shift the board the direction you're spinning, then counter-rotate it the other way and unwind back to straight for the landing.
The hard part isn't the shifts, it's keeping track of where you are in the spin while your legs are busy. Have the rotation dialed before you add anything to it.
See more here at the 3:50 mark:
Video by Ed Shreds