Double sticking looks very much like Single sticking in that the devilstick is passed back & forth between the handsticks, but instead of catching & throwing the devilstick with one handstick at a time you trap the devilstick in between both handsticks. This method of devilsticking gives you much more control & is very useful for recovering a devilstick that isn't doing exactly what you want it to do. Some people also refer to this trick as trapping.
Start off with your basic Single sticking. In single sticking you only pass the devilstick with enough force to make it do about a quarter of a turn. Try single sticking with enough force so that the devilstick makes a full half turn, at the point where you catch & throw the devilstick it should be laying flat to the ground. You'll soon see that this is much more difficult than it sounds (not impossible though). The devilstick will tend to roll forwards & backwards & over the handsticks to.
To help keep control while catching on one handstick place the tip of the other handstick onto the bottom half of the devilstick below the central waistband. The catching handstick is stopping the top half from going down & the other is stopping the bottom half from coming up. DON'T HIT DOWN, just place the handstick against the devilstick & gently push down a little. Keep it there until after you have made the throw with the other hand. This will stop the forwards & backwards motion of the devilstick. The throw is still made with the same handstick that you make the catch with (the one that touches the top of the devilstick) the other handstick is simply there for control.
Practise so that you touch the devilstick with both handsticks at the same time, you should only be able to hear one tap. The whole point of double sticking is to 'hold' the devilstick between two points rather than one. If you hear two taps then the devilstick is still only being held on one stick only, meaning it can still roll around in ways that you may not want it to.
Now despite the fact that I have just written a tutorial on double sticking I am now going to tell you to try NOT to use this trick. On its own double sticking looks no different from single sticking, so there is little point in considering it a new trick. The only advantage to double sticking is that it gives you more control & is great for rescuing an errant devilstick. It is very useful while learning new tricks, which is why I have written this page. For this reason though, whenever I see a performer going into double sticking I automatically think that they are not confident enough to only use one handstick.
But hey, I'm a control freak.