If there are two POs for the same invoice:
Adding the first PO into the PO lines tab will show the lines for that PO.
Adjusting the This voucher Quantity and Value columns, then saving will add the PO into the This Voucher tab.
Adding the second Purchase Order into the PO Lines tab will change the PO Lines view, so that only the lines on the second PO are showing.
Adding the quantity and value for the second PO and then saving will mean that both POs will show on the This Voucher tab.
The This Voucher tab is what your final invoice will look like — so these have both been added correctly.
