Dear Friend,
What I understood from your question is that you want to retrieve the initial value of the first drop-down if this is the case then you just have to save the initial value in the hidden field before changing or clearing it (in your case) and then select the same field again after binding data to the first drop down like :-
HiddenField hdnID= ddlFirst.SelectedValue;
ddlFirst.SelectedValue=hdnID.Value;
I hope this will solve your problem.
Thanks