The most important part of a successful pull-pull is to have a bit of positive Ackerman built into the setup. Then just take out all the slack when at neutral, never tight enough to be able to pluck it and get a tune. With this set up, the line not being pulled as you move off center will go slightly slack (a good thing). If you goofed up and got a bit of negative Ackerman the non pulling line will get tighter as you move off center (a very bad thing which will increase current draw and servo wear). I have found nylon coated braided wire fishing line one of the best type of line to use. Also use the ferules from the same fishing line supplier to make your connections.