Dynamically creating sub flow in Workflow Diagram

Hi All,
The business process is as follows:
In a purchase order queue, one purchase order includes multi-items; when the approver receives an order, he can approve them item by item, once an item is approved, this item should be populated to another queue. This order should be removed from purchase order queue until all the items are being approved.

How to design the workflow diagram on such scenario? After “Approve” task completed, the workflow should be split into multi-branches dynamically, which depends on the numbers of items.