You can put two lengths in - they do have a habit of twisting around each other though. Just recently it took me about 2 hours to pull 25 metres because cable had been badly run and twisted.
As you say an installed single length can be enough. The cable and another rope are attached and pulled through. The second rope is used to pull the first one back. Just leave a few metres at each end, attached to a block that is too big to be pulled into the duct.
Just check everything with OR when they visit.
M H C
taurus excreta cerebrum vincit
Edited by MHC (Fri 30-Jan-15 12:15:16)