In multiplex patterns, on some beats one hand throws more than one ball at the same time. These throws are shown by putting the numbers for the throws in square brackets. So  indicates that one hand throws two balls at the same time, one as a cascade throw and the other as a fountain throw.
To create a multiplex pattern you need to combine two patterns that repeat in the same number of beats. Any beat where both these patterns have non-zero throws will be multiplex throws in the final pattern. For the pattern to work the throw two beats before any multiplex throw must be a 2. Alternatively the throw one beat before a multiplex throw can be a 1, although these patterns will be harder to perform!
For example 504 and 420 both repeat every three beats. Both patterns have non-zero throws on the first beat so this will be a multiplex throw. The pattern produced by combining 504 and 420 is 24. There is a 2 two beats before the multiplex throw  so this pattern will work!
Some animated examples follow.