I used Crawford tan 4 thread cord for the binding on the second book. Working with two signatures at the same time was difficult at first, but became easier with each pair I added. It is important when adding a second signature through the same holes not to split the cord of the first signature. This required more attention and made the binding go slower.
Below is a photo showing the spines of the two finished black leather books. I was happy with how my first two patterns turned out.