for a double spreader rig.

Oh, and roverhi has it right.

Just stepped mine today (after 9 weeks of refit) and we did it all from the deck.

Just add a loop of line over the sling right next to the mast and cleat it at the base somewhere. This'll locate the sling at the balance point between the spreaders and not let it slide up. When you're ready to release the sling you just drop the crane hook to the deck. Your sling has to be at least as long as the distance from the first spreaders to whatever you can reach.

Detach one end and you're away.