An idea:
Create a duplicate of you current task page (the one with the two grids)
Put both datagrids inside a dataview with data source type = context and entity(path) = Task.
Put the following xpath in the datagrid on the left hand side: [TaskManager.ParentTask_ChildTask='[%CurrentObject%]']
Now you can open this new page on doubleclick in the sub-task in the first page.
I hope this helps.
Michiel