Two Way Binder - For use in non-isolated scope scenarios

twoWayBinder

    .factory('twoWayBinder', function ($parse) {
      function twoWayBind($scope, remote, local){
        var remoteSetter = $parse(remote).assign;
        var localSetter = $parse(local).assign;

        $scope.$watch(remote, function (value) {
          localSetter($scope, value);
        });

        $scope.$watch(local, function (value) {
          remoteSetter($scope, value);
        });
      }

      return {
        bind : twoWayBind
      };
    });

Code doesn't need to look at the parent scope. The expression should resolve on an inherited or shared scope. If the scope is isolated you shouldn't be using this script - use the scope : { ... } syntax provided by directives.