All you have to do is reload the dependent combobox every time the user selects something on the first.
Topics to research:
Load CComboBox Dynamically[
^]
Capture user selection in CComboBox[
^]
A couple of different approaches you can take on the dependent box:
- Leave it blank until user selects something.
- Fill in based on default choice for first.
- Don't show it until user selects something from first.