Added a method to attach "Node-style" callbacks as handlers.

test/promise-test.js

@@ -660,6 +660,38 @@ define('when/promise-test', function (require) {
 			}
 		},
 
+		'both': {
+			'should return a promise': function () {
+				assert.isFunction(defer().promise.both().then);
+			},
+
+			'should register callback as callback': function (done) {
+				var d = when.defer();
+
+				d.promise.both(
+					function(err, val) {
+						assert.equals(val, 1);
+						done();
+					}
+				);
+
+				d.resolve(1);
+			},
+
+			'should register callback as errback': function (done) {
+				var d = when.defer();
+
+				d.promise.both(
+					function(err) {
+						assert.equals(err, 1);
+						done();
+					}
+				);
+
+				d.reject(1);
+			}
+		},
+
 		'yield': {
 			'should return a promise': function() {
 				assert.isFunction(defer().promise.yield().then);

when.js

@@ -95,6 +95,21 @@ define(function () {
 			}
 		},
 
+		/**
+		 * Calls combinedCallback with (reason, value) on either fulfillment or
+		 * rejection.
+		 * @param  {function} combinedCallback handler to be called on either
+		 * fulfillment or rejection.
+		 * @return {Promise}
+		 */
+		both: function (combinedCallback) {
+			return this.then(success, combinedCallback);
+
+			function success(value) {
+				return combinedCallback(null, value);
+			}
+		},
+
 		/**
 		 * Shortcut for .then(function() { return value; })
 		 * @param  {*} value