ActionScript 3.0 :: Use Loader To Load Two Picture?

I use Loader to load two picture,such as 1.jpg and 2.jpg,like follows:

_p1=new Pic("1.jpg");_p2=new Pic("2.jpg");

Then I add the two picture into a Sprite,like follows:

Sprite _g=new Sprite();
_g.addChild(_p1);
_g.addChild(_p2);

Then I draw a circle as _g mask,such as _c
_g.mask=_c;
 
I want to realize following effect.First appear 2.jpg,and during the mask _c radius is from 0 to 300,the picture 1.jpg is appearing synchronously in mask _c.