Tweener Documentation
AS2 and AS3.
... onOverwriteParams:value, ...
value:Array — A list of parameters (of any type) to be passed to the onOverwrite function. 
// Applies two tweenings on a movieclip, warning when the first is overwritten (AS2)
onOvr = function(tName:String) {
	trace ("The tweening " + tName + " has been overwritten at " + this);
};
Tweener.addTween(myMovieClip, {_x:100, time:1, onOverwrite:onOvr, onOverwriteParams:["_x to 100"]});
Tweener.addTween(myMovieClip, {_x:50, time:1});