Can more than 1 PO line appear on a packing slip?
At this point, packing slips are held at the line level unless all items are shipped under the same ASN. In that case, all lines on the ASN would be included on the packing slip.
The way you would bulk ship items so that they share an ASN is from the main PO dashboard, choose all lines shipping together. Then fill out the shipping information for each line. (Remember, the items in the header section will apply to all items in the shipment, while the line level details are only for that line)
Once you have saved your shipment, when you look at the ASN on the Shipments Dashboard and print, all items in the shipments will appear.