If user must (or will) navigate to Doctor page directly after selecting a Location
you can send LocationID as parameter in QueryString.
Response.Redirect("DoctorPageUrl.aspx?LocationID=" + ddlLocation.SelectedValue);
And in Doctor page you will use it to populate your other drop down list.
protected void Page_Load(object sender, EventArgs e)
{
if(!this.IsPostBack)
{
if(Request.QueryString["LocationID"]!= null)
{
FillDoctorsByLocation(int.Parse(Request.QueryString["LocationID"]);
}
}
}
If that is not the case I strongly recommend
preet88's solutions.