$watch-ing variables across multiple scopes

I'm in a directive, and I would like to do a union $watch on a variable that exists on the local $scope and one that exists on $rootScope. It would look something like:

myapp.directive('cohnbread',
            ['TEMPLATE_URL', '$rootScope',
    function(TEMPLATE_URL,    $rootScope) {
        return {
            templateUrl: TEMPLATE_URL + "cohnbread.html",
            scope: { odp: '@' },
            link: function(scope, elem, attrs) {
                // here I would like to register a callback to fire 
                // whenever scope.bar or $rootScope.foo fires. Something like:
                $watchBoth('scope.bar', '$rootScope.foo', onBarOrFooChange);
            }
        };
    }]);

Is there a workaround to make this happen?

Edited to provide more info.