I was unable to find any actual documentation on how split shipments are supposed to be handled, it is more guesswork from a number of sources.
The "Flat File Shipping Confirmation" Excel template has this to say:
If a customer has ordered more than one item from you, and you are fulfilling the order in more than one shipment, then order-id, order-item-id, quantity and ship-date are all required for each shipment that you are confirming.
It is my understanding that most XML feeds are actually handled by the same routines as flat files after they have passed validation. So I assume the XML feed should be created in accordance with above quoted requirement to achieve the same goal - which is exactly what you posted as possible solution. Please note that every <OrderFulfillment>
can have multiple <Item>
elements which allows you to specify which quantities of which items are included in which package.