Master With Two Detail Views

Hello community,
I think my problem is easy to solve even though I did not find a solution through different tutorials and help pages. Here it is (select statements are hier simplified):
In the gridview "GridView1" I have a master record with person_id, which is the data-key-value. There is also another id-field named task_id (This record comes from a database view which joins the persons- and the tasks- table)

                SelectCommand="SELECT [id], [person_id], [task_id] FROM [ViewPersonTasks] WHERE ([id] =
@id)"
For both fields I want to display details in two different detail-views. One for the person (depending on person_id) and  one for the tasks (depending on the task_id).
The first one is easy. I declare a details-view for the person data based on a SqlDataSource with a control-parameter like this:
                SelectCommand="SELECT [person_id], [first_name], [last_name], [birth_date] FROM [TabPersons] WHERE ([person_id] = @person_id)"
               ....

                <SelectParameters>
                    <asp:ControlParameter ControlID="GridView1" Name="person_id" PropertyName="SelectedValue" Type="Int32" />
                </SelectParameters>
But now the problem: how should I declare a parameter @task_id for the task_id, so that the second select statement for the tasks-details-view retrievs the data for the tasks:
 
                SelectCommand="SELECT [task_id], [task_name],
[task_date], [task_description] FROM [TabTasks] WHERE ([task_id] =
@task_id)"
@task_id should have the value from the task_id-field of the master record, displayd in the master grid-view.
Thank you in advance for your help