Double stick isn't something that I practice or teach much. It's highly unlikely that you'll ever find yourself in a situation where you have two sticks against an attacker. Double sticks can be used to represent two blunt objects, two sharp objects, or a combination of the two, but if I'm going to use a weapon I generally prefer to have one hand free. With that said, double stick practice can be fun, and it's great for coordination.
Below you'll find a series of instructional images from my upcoming book on weapon use and defense, demonstrating a simple double stick "pattern" that can be used with the 4 Step Matrix. Notice that the first two strikes can be used as a covered entry, the second and third pairs are covered follow ups to the arm and head, and the final two represent a covered exit. The backward step or return that coincides with the fourth pair of strikes could be used in a self defense situation, but other exiting footwork would generally be better. However, returning to the original starting position allows the drill to continue repeatedly. This pattern can be practiced with a training partner doing the identical pattern across from you, hitting sticks, but one of you will need to change the angle of the horizontal blows in order to avoid hitting each other.
You can see the same patterns with the addition of the next two series of the 4 Step Matrix using a single stick here.
Below is an example of a double stick application. This application is obviously staged to show the use of the above training pattern. In a real self defense situation the pattern may or may not work exactly as demonstrated.
See more functional kali / eskrima footwork and stickwork: