how to control one movieclip from another?

[pzb4]

I have 2 movieclips on the stage. When i get to a certain frame in one movieclip, I want it to tell the other movieclip to stop. How do I do this? Let's call them mc_1 and mc_2. On frame 10 of mc_1 I was trying to call:

MovieClip(this.parent.mc_2).stop();

but I get an error that says "1119: Access of possibly undefined property mc_2 through a reference with static type flash.displayisplayObjectContainer."

Both movieclips are on the stage in the same frame. Can someone help? This should be simple but I don't know what I'm doing wrong.