Layered objects and functions

I have two movie clips on the stage: mcA and mcB, each have a function when rolled over:

mcA.addEventListener(MouseEvent.ROLL_OVER , aActivated);
function aActivated (event:MouseEvent):void
{
     trace('mcA was rolled over');
}

mcB.addEventListener(MouseEvent.ROLL_OVER , bActivated);
function bActivated (event:MouseEvent):void
{
     trace('mcB was rolled over');
}

Under certain circumstances mcA may overlap mcB. When the cursor rolls over their intersecting area I would like both functions to launch, how do I do that? Currently, the problem is that only the object on the top will launch its function.

Thanks for the help.