如何从wpf中的另一个窗口访问一个窗口的控件（richtextbox）？

I'm sure this is something very simple but I can't figure it out. I've searched here and on msdn and have been unable to find the answer. I need to be able to set the richtextboxes selection via richtextbox.Selection.Select(TextPointer1, Textpointer2).I'm sure this is something very simple but I ca