You can do this by setting AutoPostback to true on your first dropdownlist
and handling the OnSelectedIndexChanged event. Within this code, you can
update the other dropdownlist. Here is some example code that I used to
update label controls based on dropdownlist. You should be able to do the
same with your secondary DDL:
protected void DropDownListItem_SelectedIndexChanged(object sender,
EventArgs e) {
int itemID =
int.Parse(((DropDownList)GridViewMain.FooterRow.FindControl("DropDownListItem")).SelectedValue);
item = new ItemData().Get(itemID);
((Label)GridViewMain.FooterRow.FindControl("LabelCode")).Text =
item.Code;
((Label)GridViewMain.FooterRow.FindControl("LabelDescription")).Text =
item.Description;
((Label)GridViewMain.FooterRow.FindControl("LabelNote")).Text =
item.Note;
}
Hope this gets you going in the right direction.
