It does not appear to be too difficult?!

On a knee jerk reaction I fired up VS and drop a gridview and a
sqldatasource.  I used stuff that already exists in the Northwind database.  
(Take advantage of this you learn a lot this way.)  

I notice there was a store procedure called CustOrderHist with one parameter
so it parallel what you were doing closely.  I gave it a default fired up the
page and it worked with out a hitch.  

The declarative stuff:

   <div>
       <asp:GridView ID="GridView1" runat="server"
AutoGenerateColumns="False"
           DataSourceID="SqlDataSource1">
           <Columns>
               <asp:BoundField DataField="ProductName"
HeaderText="ProductName"
                   SortExpression="ProductName" />
               <asp:BoundField DataField="Total" HeaderText="Total"
ReadOnly="True"
                   SortExpression="Total" />
           </Columns>
       </asp:GridView>
   </div>
   <asp:SqlDataSource ID="SqlDataSource1" runat="server"
       ConnectionString="<%$ ConnectionStrings:NorthwindConnectionString %>"
       SelectCommand="CustOrderHist" SelectCommandType="StoredProcedure">
       <SelectParameters>
           <asp:QueryStringParameter DefaultValue="ALFKI" Name="CustomerID"
               QueryStringField="custid" Type="String" />
       </SelectParameters>
   </asp:SqlDataSource>

If this does not help let me know I will take a closer look for you.

Wow!  

I played a little more with what you were talking about and did ran accross
a similar problem.

I came up with an approach where you can use the grids alone and it will
work without the use of the textbox.  I could not get the textbox to work but
I have a hunch...  In the mean time this might work.

   <div>
       <table class="style1">
           <tr>
               <td>
                   <input id="txtCustid" type="hidden" runat="server"
value="ALFKI" />
               </td>
               <td>
                    </td>
           </tr>
           <tr>
               <td>
                   <asp:GridView ID="GridView2" runat="server"
AutoGenerateColumns="False"
                       DataKeyNames="CustomerID"
DataSourceID="SqlDataSource2">
                       <Columns>
                           <asp:CommandField ShowSelectButton="True" />
                           <asp:BoundField DataField="CustomerID"
HeaderText="CustomerID" ReadOnly="True"
                               SortExpression="CustomerID" />
                           <asp:BoundField DataField="CompanyName"
HeaderText="CompanyName"
                               SortExpression="CompanyName" />
                       </Columns>
                   </asp:GridView>
                   <asp:SqlDataSource ID="SqlDataSource2" runat="server"
                       ConnectionString="<%$
ConnectionStrings:NorthwindConnectionString %>"
                       SelectCommand="SELECT [CustomerID], [CompanyName]
FROM [Customers] ORDER BY [CompanyName]">
                   </asp:SqlDataSource>
               </td>
               <td valign="top">
       <asp:GridView ID="GridView1" runat="server"
AutoGenerateColumns="False"
                       DataSourceID="SqlDataSource1">
           <Columns>
               <asp:BoundField DataField="ProductName"
HeaderText="ProductName"
                   SortExpression="ProductName" />
               <asp:BoundField DataField="Total" HeaderText="Total"
ReadOnly="True"
                   SortExpression="Total" />
           </Columns>
       </asp:GridView>
   <asp:SqlDataSource ID="SqlDataSource1" runat="server"
       ConnectionString="<%$ ConnectionStrings:NorthwindConnectionString %>"
       SelectCommand="CustOrderHist" SelectCommandType="StoredProcedure">
       <SelectParameters>
           <asp:ControlParameter ControlID="GridView2" DefaultValue="ALFKI"
               Name="CustomerID" PropertyName="SelectedValue" Type="String"
/>
       </SelectParameters>
   </asp:SqlDataSource>
               </td>
           </tr>
       </table>
   </div>

I had to make sure you had a datakey on the first table and added a select
command link button then used the wizard to bind datasource1 to the
selectedvalue of Gridview2 and that worked.  But the first time it did not
work even though I had manually selected datakey field.  I re-did it this
time let the wizard do it and it worked.  I could not see the difference!! I
will keep looking to see if I can spot why this worked only when I let the
wizard generate the datakeys.

I hate this declarative stuff.