Automatically Fill a Second Term Reference Field in a Node Based On Filled Value from a First Term Reference Field
Have look at the following modules

Reference field option limit


  This module allows reference fields of several types to have their widgets' available options limited by the values of other fields in the current entity.


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.