With dual sheets there is a clever system where you twine the middle of the sheet in to a loop with a short second piece of sheet. The loop goes through the sail and the extra piece of sheet goes through the loop it preventing the loop from sliding out of the sail. To secure this knot, a plastic stopper at the end of the short piece of sheet ensures it won’t slide out of the loop. With the loop tight the stopper cannot pass the loop. Perhaps it is better explained by a picture, have a look at the third picture (scroll down) here: Wayfarer Institute of Technology (WIT)
I’m using this method on the jib sheets and the main halyard – works a treat. Beats tying two bowlines every time I sail, anyway! Whatever I do I just can’t get my head around that knot.