What happens when you combine two disordered systems? You get order. The key here is that the systems have to be exactly that: systems. That is, the parts have to be interactive. If you simply put two piles together, you get a bigger pile, just as disordered (well, twice as disordered). But when the parts interact, you have a system, and those systems can be disordered.
More than that, the disordered systems attracted each other. An ordered system also did not increase order. Rather, the disorder remained.
Think about what this means for spontaneous orders of all sorts. We can understand such systems as being, essentially, disordered. What gives them order? The typical person would insist that one must introduce order to get order. But we are increasingly understanding that this is hardly the case. Another disordered system would be more likely to introduce order.
Take for consideration our monetary system and the economy. We have a central bank whose job it is to create order in the economy by creating order in the monetary system. However, what we actually see is the economy becoming less stable the more stable/predictable the monetary order is. A chaotic free banking system would in fact be the kind of system that would in fact stabilize the economy, making it more orderly.
Of course, the authors don't discuss what happens if you add a third disordered system to the mix.