Some info on this demo.

The original carousel version of the Slick slider uses two sliders to implement the carousel. This version uses a single slider which is cloned and massaged to display it as a thumbnail track under the original slider.

There are benefits and disadvantages to this...

1. Pro:You only need one slider in the markup and one initialization call!
2. Con:We have to process the markup via javascript which then calls init again in order to display the second slider.
3. Pro:The sliders are related but do not force navigation on each - meaning if you slide the thumbnail slide, the top slide doesn't auto-advance.