A double binder is what it sounds like: two binder leaves instead of one. Rollers use it when a single capote will not hold the bunch, when the blender wants a particular burn, or when a second leaf is part of the recipe rather than a repair.
You will not see it from the outside. The wrapper still covers everything. Construction notes on a band or a factory tour are how you know.
It is not a substitute for entubado or for extra filler. It is one more sheet in the sandwich.