refer to all the articles, complete with pictures, on setting up Pull-Pull systems available under =
"Aircraft Design. Airfoils, Pull-Pull Systems, Flight Trim & Static Thrust Calculator. (CG,Colour,Trim etc.= more in Folders below)"
at
http://homepages.ihug.co.nz/~atong/
The cables should be taught but not tight so that when one side is pulling, the other has a slight amount of slack. This to relieve any pressure on the servo or hinges if the symetry is not true.
regards
Alan T.