Tim,
Have fun!
Mike
You could use a wrapper entity that is set in a view arround both the grid with POHead objects and the POLines templateGrid.
On your POHead grid you add a action that is the default action when a user single clicks a POHead. The microflow that gets called by the button sets an association to the wrapper. NB: Make sure you also set remember selection in the POHead datagrid.
Now you can also select lines on the POLines templategrid and use an action button on that grid to set the POLines on the selected POHead. By passing in the POLines selected and the wrapper object. Since the wrapper entity has an association set to the selected POHead you can retrieve that and set the selected POLines on that POHead.
The wrapper object can just be a non persistend entity because you won't need to store the temporary data in the database.