I've wondered about that too. Yes, LE used the singular, "water bladder", and did not specify who was carrying it. Could be either one of them, you can add one to a baby backpack, one by Osprey says "Pocketing provides storage for hydration reservoirs, layers, snacks, and more" for example. You can easily remove them from an old pack, or buy just the bladder with hose to add it to another pack. Perhaps they distributed the load by having EC carry the water. Last possibility, they did both have them, they usually are an individual item. I tried one, didn't like it for several reasons. One disadvantage is you can't readily judge how much water you have left- imagine a 3L bladder, vs three clear 1 liter bottles. You know exactly how fast you are going thru your water with the bottles.