Multiplex means throwing more than one ball together from one hand. This tutorial is restricted to throwing two balls at a time. In siteswap notation, multiplex throws are marked with brackets, for instance, this lesson uses the throw [4x4]. With some balls, multiplex juggling is relatively easy, but with for instance large balls, it is impossible. In that case, you just have to skip all lessons with multiplex throws.
Step 1 Put two balls in the same hand side by side. Place your middle finger between the two balls so that the balls will separate when thrown. When throwing, keep your wrist relatively loose but fingers relatively stiff. This will help making the throw consistent. Try to get the balls equally high and well separated. Learn both sides in each step.
Step 2 Two throws.
Step 3 Three throws.
Step 3 Four throws (adding one to the front). Now you will have to arrange the multiplex hand for the throw quickly. Catch the first ball in your fingers and second one in your palm, and then quickly move them side by side such that the first one touches your thumb, index and middle fingers, and the second one your middle finger, ring finger, and pinky.
Step 4 You are done. In order to not get confused, note that each hand has three throws in a row.
Mixing This pattern mixes easily with synchronous patterns such as the shower here.
Four-ball multiplex pattern in Lesson 33 is not much more difficult than this. If you had no difficulty here, you might want to skip ahead and try that immediately.