<asp:DropDownList ID="ddlLista1" DataSourceID="dtsLista1" AutoPostBack="true" DataValueField="IDUtente" DataTextField="Username" runat="server" />
<asp:DropDownList ID="ddlLista2" DataSourceID="dtsLista2" DataValueField="IDUtente" DataTextField="Username" runat="server" />
<asp:SqlDataSource ID="dtsLista1" ConnectionString="Server=XXX.XXX.XXX.XXX;Database=AAAAAAAAAAA; Uid=AAAAAAAAAAA;Pwd=AAAAAAAAAAA;" ProviderName="System.Data.SqlClient" SelectCommandType="Text" SelectCommand="SELECT IDUtente = -1, Username = '' UNION SELECT TOP 10 IDUtente, Username FROM dbo.Utenti" runat="server" />
<asp:SqlDataSource ID="dtsLista2" ConnectionString="Server=XXX.XXX.XXX.XXX;Database=AAAAAAAAAAA; Uid=AAAAAAAAAAA;Pwd=AAAAAAAAAAA;" ProviderName="System.Data.SqlClient" SelectCommandType="Text" SelectCommand="SELECT TOP 10 IDUtente, Username FROM dbo.Utenti WHERE IDUtente = @IDUtente" runat="server">
  <SelectParameters>
    <asp:ControlParameter Name="IDUtente" ControlID="ddlLista1" PropertyName="SelectedValue" />
  </SelectParameters>
</asp:SqlDataSource>