This is sometimes called a master detail relationship.
You should have a field in your orders table that uniquely identifies each order record such as an auto-entered serial number field. I'll call it OrderID.
You should already have this relationship for filling in the lineitems for an order:
Orders::OrderID = LineItems::OrderID
If you have all that, you can define this relationship:
ShippingCenter::gSelectedOrder = SelectedOrderLineItems::OrderID
Where gSelectedOrder is a field with global storage specified and SelectedORderLineItems is an occurrence of the lineitems table created by selecting LineItems in Manage | Database | relationships and clicking the duplicate button (Two green plus signs). You can then double click the new occurrence to open a dialog box where you can rename it as I have here.
Then you can either put a button in the Orders portal or select the fields in the portal row and use button setup to turn the entire row into a button. Either way, have the button perform this script:
Set Field [ShipingCenter::gSelectedORder ; OrdersPortal::OrderID]
Make your second portal a portal to SelectedOrderLineItems.
Wow, Amazing. Never knew you can do that between portals.
Worked like a charm.
Thanks again PhilModJunk.
Very greatful for all your help!
The next step would be to use conditional formatting with this type of expression:
ShipingCenter::gSelectedORder = OrdersPortal::OrderID
To highlight the most recently clicked portal row in your OrdersPortal so that you know which order's line items are currently displayed.