You repeat the item structure (and everything below it) as often as you have PO's. Two PO's will result in two item (and everything below it) structures in the message. Just copy the item structure in SOAP UI like:
<Item>
<PO number></PO number>
<field 2></field2>
<field 3></field3>
</Item>
<Item>
<PO number></PO number>
<field2></field2>
<field3></field3>
</Item>
And fill them with values