- Invoice account
or - automatic summary
Are the two POs for the same Vendor? If so, choose 'Invoice account', and then hit 'Arrange'.
Or, on the Vendor (using the 'Summary update' button on the 'Supplier' tab -> 'Setup area' on the ribbon bar), set Automatic summary for Product Receipts/Delivery note and you can use 'Automatic' on the PO Receipt dialogue.
If the POs are for different Vendors, you can't. Hopefully for obvious reasons!