The "Toss 'n Go" method is the fastest, easiest, and safest way to rig a retrievable rappel rope in most situations. To rig the toss 'n go method:
There are no complicated knots that could go wrong. Once you get to the bottom, you can easily pull one end of the rope to retrieve your rope. More friction through the rappel device is advantageous for heavy rappellers or thin ropes.
Light rappellers, thick ropes, or tall cliffs may introduce so much friction into the system that it becomes difficult for the rappeller to go down. For example, a 140-lb person using an ATC on two strands of 11mm rope will hang at the top of a 60-foot cliff and will have to push the rope through the ATC in order to go down.
Pulling half the rope through the anchor can create rope grooves in the rock.
Both strands must reach to the ground.
Rappelling double strand doesn't work if there is a knot on either strand.
It is very difficult to ascend two strands of rope.