Swapping movieclips or something help

alrighty what im trying to do which i bet im overlooking something simple is:

i have an empty movie clip which is following the mouse, a custom cursor container…
what i want to do is load 1 of 2 movie clips into it depending on whether its on the right half of the stage or the left hand…i have a black arrow and a white arrow (ba_mc and wa_mc)…
so when the cursor passes the middle of the stage to the left i want it to load the wa_mc and when it passes the middle of the stage to the right i want it to load the ba_mc…

heres the code i was trying among other things…

////////////////cursor code////////////////////////
Mouse.hide();

/////ac_mc is used as a container mc for the custom cursor
/////sets the x and y of the custom cursor container
ac_mc._x = _xmouse;
ac_mc._y = _ymouse;

/////attaches the custom cursors to the cursor movie clip
ac_mc.attachMovie(“whiteArrow_mc”, “wa_mc”, 0);
ac_mc.attachMovie(“blackArrow_mc”, “ba_mc”, 1);

/////reupdates the x and y of custom cursor container to follow the mouse
ac_mc.onEnterFrame = function()
{
ac_mc._x = _xmouse;
ac_mc._y = _ymouse;

}
//makes blackarrow visible when on right side of stage…
if(_root.ac_mc._x >= Stage.width/2)
{
this.ba_mc._visible = true;
}
//makes blackarrow invisible when on left side of stage…
else if(_root.ac_mc._x <= Stage.width/2)
{
this.ba_mc._visible = false;
}

as i said before im sure its something small that im overlooking but ne help would be highly appreciated