There are many issues with that suggestion Geoffers, not only would you need two winches but two winch operators (which one is giving the con to the pilot?) The second door would use up valuable cabin seating space so you would need a bigger aircraft as well.
The main problem is that a winching scenario where you could winch from both sides is unlikely as you would be hovering right over the vessel. If it is a large ferry and it is not on its side it is probably quicker to try and land as many have a large flat area on top. With a liferaft the downwash would present a major problem if you were trying to hover over it and you would need 2 winchmen to prepare the casualties for the 2 winchops as they would be on opposite sides of the liferaft.
One way of speeding up the process is to have 4 strops so the winchman on the ground/deck can already be preparing the next 2 evacuees as the winchop is bringing 2 into the cabin - this was used successfully at Boscastle instead of the single strop lifts the RN did.