simply,
manually write dropdown selectindexchanged event in sourcefile and .cs file and set autopostback=true.
then in selectedindexchanged event use sender object to retreive combobox selectedindex and text and use dropdownlist clientid to get selected row like
DropDownList ddl = (DropDownList)sender;
string s = ddl.ClientID;//use substring to retreive rowindex
if (ddl.SelectedItem.Text == "Your selected text")
{
dropdown1.Items.FindByText("Your text"].ToString()).Selected=true;}