A double binder means exactly what it says: two binder leaves instead of one. Rollers use it when a single capote (binder leaf) will not hold the bunch together, when the blender wants a particular burn, or when the second leaf is part of the recipe rather than a repair.
You cannot see a double binder from the outside, because the wrapper still covers everything. You find out about it from information on the band or on a factory tour.
It does not replace entubado bunching or extra filler. It is simply one more layer of leaf.