as3 如何判断同时按下两个键

var key_obj: Object = new Object;
function onKeyDownHandler(_evt: KeyboardEvent): void {
	var _code: String = String.fromCharCode(_evt.keyCode);
	if (key_obj.hasOwnProperty(_code)) {

	} else {
		key_obj[_code] = true;
	}
	if (_evt.ctrlKey && _code == "S" && key_obj[_code] == true) {
		key_obj[_code] = false;
		trace("Ctrl键被按下", _code);
	}
	_evt.altKey && trace("Alt键被按下");
	_evt.shiftKey && trace("Shift键被按下");
	//for (var p:String in key_obj) {//用于其它字母和数字鍵
	//        trace("当前按下的键有："+p);
	//}
}
function onKeyUpHandler(_evt: KeyboardEvent): void {
	var _code: String = String.fromCharCode(_evt.keyCode);
	key_obj[_code] = null;
	delete key_obj[_code];
}
function missingStage(e) {
	trace("DEACTIVATE");
	key_obj = new Object;
}

function getingStage(e) {
	trace("ACTIVATE");
	/*stage.stageFocusRect = false;

	stage.focus = stage;*/
}

stage.addEventListener(KeyboardEvent.KEY_UP, onKeyUpHandler);
stage.addEventListener(KeyboardEvent.KEY_DOWN, onKeyDownHandler);
stage.addEventListener(Event.DEACTIVATE, missingStage);
stage.addEventListener(Event.ACTIVATE, getingStage);