Dual Line Preventer Set-Up
A dual line set-up, while more complex has a few benefits over the single line set-up. To set it up the Dual Line system, you have to run a line preferable from the end of the boom forward to be attached with a snap shackle to the preventer attachment at the boom. Additionally, on either side of the boat there needs to be a line run from the side of the cockpit via a clutch and rollers to a strong point ahead of the mast. (see also Dual Line Preventer drawing)
Advantage: Once you are on a downwind or near downwind course you open the clutch near the cockpit of the lee line and go forward to the end of this line at the strong point ahead of the mast. Pulling out the line and then grabbing the snap shackle at the preventer attachment point from the line on the boom (which should be easily reachable). You clip them together, walk to the back and tighten the preventer line.
To avoid the issue of having to feed the line attached to the boom around the mainsheet tackle when gybing, you can set up to lines on the boom, one to port and one to starboard avoiding the hassle of having to feed it around the mainsheet tackle.
Now what about multipurpose use of this system?
Well, having the line on the boom firmly attached to the preventer attachment point serves has a great dual purpose as jackline, when the boom is winched into the mid ship position. Also, on a boat with a selftacker, either one of the side lines can be used to act as a barber hauler or to fix the selftacker car to one side, should one consider to heave to.
On my own boat, I also use the windward line to control the tackline of my Asymmetrical Spinnaker from the cockpit.
Disadvantage: More complex set-up with multiple lines required.
Advantage: Once you are on a downwind or near downwind course you open the clutch near the cockpit of the lee line and go forward to the end of this line at the strong point ahead of the mast. Pulling out the line and then grabbing the snap shackle at the preventer attachment point from the line on the boom (which should be easily reachable). You clip them together, walk to the back and tighten the preventer line.
To avoid the issue of having to feed the line attached to the boom around the mainsheet tackle when gybing, you can set up to lines on the boom, one to port and one to starboard avoiding the hassle of having to feed it around the mainsheet tackle.
Now what about multipurpose use of this system?
Well, having the line on the boom firmly attached to the preventer attachment point serves has a great dual purpose as jackline, when the boom is winched into the mid ship position. Also, on a boat with a selftacker, either one of the side lines can be used to act as a barber hauler or to fix the selftacker car to one side, should one consider to heave to.
On my own boat, I also use the windward line to control the tackline of my Asymmetrical Spinnaker from the cockpit.
Disadvantage: More complex set-up with multiple lines required.
Line leading forward from winch and clutch
Boom line 
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Line leading forward
Preventer hooked up and in use 
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Boom line attached to vang with snatch shakel
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