How do you juggle devil sticks?

The trick is not to hit the baton (central stick). You want to catch it with the handstick and toss it back so the key is absorbing the impact. Just start by trying to go back and forth with quarter spins (often called a "Tick-Tock") while working to keep the baton on a steady plane.

That being said, it would be almost impossible to explain through text alone. A number of great tutorials are available on YouTube.