Solution : https://service.sap.com/sap/support/notes/181377 (SAP Service marketplace login required)
Key words : 
create multiple assigned purchase orders, assign shopping basket items, additional key words bbppu01, multiple account assignment, add field 'po_item', data element 'ebelp', product sap business-, sap business-, e-business, cost objects
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| 185164 | SAP Bus.-to-Bus. 2.0A Installation: Composite Note |