Impossible to give precise advise without a concise-but-complete code
example. But the general idea is for your second DataSet (the one bound to
"combo2") to be populated based on the selection of the first ComboBox
("combo1").
Depending on how the binding is configured, you should be able to re-query
the second DataSet according to changes in the selection in "combo1", and
have the new rows in the second DataSet reflected automatically in
"combo2".
Pete