You should be able to achieve this w/ PartialTriggers (FYI, I use Apache Trinidad). Here is a snippet of my JSF code:
First ListBox:
Second ListBox:
Notice the second listbox: I have the partialTriggers attribute set to the id of the first listbox and in the backing bean whenever the user selects a value in the first listbox, this valuechange is captured and sent to populate the second box.
Hope this helps.