With twin cables you should always replace both at the same time. As for why one is pulling more than the other, you must ensure the cable is fitted through every holder on its course to the brake. If not it will not be able to pull equally. If they are fitted exactly the same the adjusters need adjusting (logically) but it is well worth buying new adjusters for new cables. A fitting kit for each rear brake will include adjusters and new return springs. An old trick we used to do which may work on older cars was to apply the hand brake in reverse! This set the adjusters for new cables but don't quote me on it working for newer cars or Fiats. Just took mine to the local tyre dealers who are superb at brakes to ensure its done right. Remember if your foot brake fails, its your last chance so well worth getting it done sooner rather than too late.