Single ply vs 2-ply jute
What's the difference?

The smart ass answer is 1-ply has one ply per yarn and 2-ply has two. That doesn't help you much though. Let's see if I can get through a few technical terms to get us on the same page and then see if my answer helps you.

Each of our ropes is made up of three strands twisted together. Each strand is made up of a number of yarns. Each yarn can have one or more plies. Our hemp yarn, for example, has three plies per yarn which is 17 yarns per strand making 153 plies per 6 mm rope.

Our jute rope comes in two flavors. The single ply 6 mm jute ropes have three strands, 17 yarns per strand and each yarn has a single ply. This makes for 51 plies total. It should be noted that the single ply of this yarn is as thick as a hemp yarn having three much smaller plies.

Our two ply 6 mm jute rope is three strands, 3 yarns per strand, 2 (much thicker) plies per yarn for a total of 18 plies in the rope.

Because of the smaller yarns, the 1-ply rope is much smoother than the 2-ply. The larger yarns make bigger "bumps" in the surface of the rope. Imagine the difference in the overall surface of a line of basket balls as opposed to baseballs. The surfaces of both ropes smooth out over time and the difference becomes less as the ropes are broken in.

The larger yarns of the 2-ply means there are larger pockets of air between the yarns. This equates to the rope being resistant to "packing" or becoming dense. The air spaces keep it more pliant over time.

The larger 2-ply yarns are stronger. If pressure is applied against yarn, it is less likely to deform or break. Jute rope can, and will, become deformed with use and requires maintenance to keep in good shape. Thus the 2-ply is more forgiving.

The smaller 1-ply yarns move together better due to their close proximity.

In short, the 2-ply rope is hardier and will last longer. The 1-ply rope is lighter feeling and more responsive.

It's a matter of preference.