What is a double envelope?

The flanks of the opponent are attacked simultaneously in a pinching motion after the opponent has advanced towards the center of an army which is responding by moving its outside forces to the enemy's flanks, in order to surround it.

At least from tactics, techniques, and procedures I've picked up it could mean vertical envelopement (ingress into the top of a building from a rotory wing bird) and a ground ingress from a ground combat element. The effect would put a squeeze on the enemy in the center of the building. The only problem is deconfliction of fires, what with multiple elements on different levels of the target building, and comm and signal plan between elements. I may be wrong though. Please correct me if so.