Double warp beam consists of one warp beam, one back beam and a pair of mounting blocks. It is located just above the regular back beam.
A double warp beam is used when you have different fibers or yarns in your warp. In some techniques certain warp threads are bound less frequently than others and therefore not woven in as much, which makes them looser after awhile. This is a good reason to have them on a separate beam.
A double warp beam also comes in handy using supplemental warp threads.