One possible solution is to save keyCodes in an array and check which keys was pressed after a short time
stage.addEventListener(KeyboardEvent.KEY_DOWN, keyDownHandler);
var timer:Timer=new Timer(100);
var tmpKeys:Array=[];
timer.addEventListener(TimerEvent.TIMER,reportKeyDown);
function keyDownHandler(e:KeyboardEvent):void{
trace(e.keyCode);
tmpKeys.push(e.keyCode);
timer.start();
}
function reportKeyDown(e:TimerEvent):void{
if(keysAreDown([Keyboard.N,Keyboard.P])) //check for simultaneous pressing of 'n' and 'p'
trace("yesss!");
else
trace("nooo!");
tmpKeys=[];
timer.stop();
}
function keysAreDown(keys:Array):Boolean{
for each(var key:int in keys)
if(tmpKeys.indexOf(key)==-1)
return false;
return true;
}