Load other table's record to portal tool

That's a bit confusing here.

It would seem your portal on the Table A layout would be a portal to Table B. How could you set up a portal to Table B on a layout based on Table B?

If you base your second layout on Table A instead of Table B, then you should be able to add a portal to Table B much like you have for you current Table A layout, but with the fields needed for this different layout. Since both layouts would be based on the same occurrence of Table A, changing layouts from one to the other would show the exact same list of Table B records in the portal.

Another option is to not use a portal on the Table B layout, but use a script that performs a Find or Go To Related Records to pull up the Table B records listed in the portal as a found set of Records for Table B. This would then allow you to use List or Table View to list the records from the portal on your Table B based layout.

Why do you need separate line item tables? Couldn't you use the same set of records for both? In which case you'd just need to create a record in Table B and set it with the same ID number used in the relationship for Table A's portal. You might not even need a Table A for purchase Orders and a Table B for receiving that purchase order.

I suppose the issue could be that a single PO could generate multiple shipments received records in cases where the vendor cannot fulfill the order in a single shipment. Is that the case here?

I'm still not convinced that copying the records is the best option here, but you can bring up a found set of line item records using either a find or go to related records--either way you bring them up on a layout based on the portal's table. Then you can either use Import Records to import the data in one step into the second table's line item table or you can loop through the set and and create a matching line item record one at a time, passing data from one table to the other in variables.

A single line item table could still work if you added a date received field to it.  That way you could filter a portal to the table based on whether there is a date added or not to show what you wanted.  You could sort them so that the ones with blank dates would show up on top, do conditional formats to make the received items different colors from the pending, etc

To Add to Mark's post (It's how I would do it here), you can add a field to identify which shipment the PO's item was received in. This can be the serial ID of  your Table B record which would then display only the received items for that shipment as selected from the original list on your PO.