var a = false,
    b = false;
$scope.$on("eventA", function () {
   a = true;
   if (b)
       performTask();
});
$scope.$on("eventB", function () {
   b = true;
   if (a)
      performTask();
});
var performTask = function () {
     ................
};

var a = false, b = false;
$scope.$on("eventA", function(){ a = true; });
$scope.$on("eventB", function(){ b = true; });
$scope.$watch(
   function() { return a && b; },
   function(newval, oldval) {
      if (newval) { performTask(); }
   }
);

var events = { a: false, b: false };
$scope.$on("eventA", function(){ events.a = true; });
$scope.$on("eventB", function(){ events.b = true; });
$scope.$watch(
   function() {
      var result = true;
      for (var key in events) {
         result = result && events[key];
      }
      return result;

   },
   function(newval, oldval) {
      if (newval) { performTask(); }
   }
);