How to make one select choice effect another

System

I have two select boxes on my DCT, both of which return a list of items from the DB via an inlinecallout script.
When the user selects an item in the first select box I would like that to determine what is read from the DB into the second select box.
I've tried using FormAPI and that works fine as far as detecting the user's selection in the first box and then calling a function. But I need this function to set the value of the second box to a new inlinecallout function with different parameters so that I can get a different list back.

Any Ideas anyone ?

Thanks

/nick