Watch multiple $scope attributes

Beginning with AngularJS 1.1.4 you can use $watchCollection:

$scope.$watchCollection('[item1, item2]', function(newValues, oldValues){
    // do stuff here
    // newValues and oldValues contain the new and respectively old value
    // of the observed collection array
});

Plunker example here

Documentation here

Starting from AngularJS 1.3 there's a new method called $watchGroup for observing a set of expressions.

$scope.foo = 'foo';
$scope.bar = 'bar';

$scope.$watchGroup(['foo', 'bar'], function(newValues, oldValues, scope) {
  // newValues array contains the current values of the watch expressions
  // with the indexes matching those of the watchExpression array
  // i.e.
  // newValues[0] -> $scope.foo 
  // and 
  // newValues[1] -> $scope.bar 
});

$watch first parameter can also be a function.

$scope.$watch(function watchBothItems() {
  return itemsCombinedValue();
}, function whenItemsChange() {
  //stuff
});

If your two combined values are simple, the first parameter is just an angular expression normally. For example, firstName and lastName:

$scope.$watch('firstName + lastName', function() {
  //stuff
});