With regards to the dual hoist; on the S92 you have an inboard and outboard hoist. On the AW139 (not sure about 189) you have a fore and aft mounted hoist. The are identical hoists, but are not /cannot be operated simultaneously. If one hoist were to fail (not cable snap) with, for instance 250' of cable out, the winchop attaches the serviceable hoist to the u/s cable via a karabina, and winches out delivering the 'S' hook to the winchman, who then connects himself to the new hook, unhooks from the other and is recovered to the a/c. The other cable is then either jettisoned or recover by hand to the a/c.
This is basic HHCO, hook hoist changeover, and would be carried out if after considering whether it would be prudent to return the winch man on a fixed cable to the deck or cliff etc, where they have just come from.
Hope I'm not teaching you to suck eggs - no offence intended.