Update Two select box when change another select box
Try one of the following module

Simple hierarchical select


  Simple hierarchical select defines a new form widget for taxonomy fields to select a term by "browsing" through the vocabularies hierarchy.


Hierarchical Select


  This module defines the "hierarchical_select" form element, which is a greatly enhanced way for letting the user select items in a hierarchy.


Or you can also achieve that by writing a custom module :)