You may have to use cascading dropdown functionality to achieve this. Is the
data for these 2 dropdowns coming from 2 different lists? If so, make sure
you include look up columns appropriately. The following link might help you
achieve this using jQuery.
http://spservices.codeplex.com/wikipage?title=$%28%29.SPServices.SPCascadeDropdo\
wns&referringTitle=Documentation