Here is a sample code for your requirement, you can modify it as per your requirement -
<asp:DropDownList ID="DropDownList1" runat="server" Width="150px" AutoPostBack="true" onselectedindexchanged="DropDownList1_SelectedIndexChanged">
</asp:DropDownList>
<asp:DropDownList ID="DropDownList2" runat="server" Width="150px" >
</asp:DropDownList>
on page load -
if (!IsPostBack)
{
DropDownList1.Items.Add("");
DropDownList1.Items.Add("Test1");
DropDownList1.Items.Add("Test2");
DropDownList1.Items.Add("Test3");
DropDownList1.Items.Add("Test4");
DropDownList2.Items.Add("");
DropDownList2.Items.Add("Test1");
DropDownList2.Items.Add("Test2");
DropDownList2.Items.Add("Test3");
DropDownList2.Items.Add("Test4");
}
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
DropDownList2.Items.Remove(DropDownList2.Items.FindByValue(DropDownList1.SelectedValue));
}