Thanks for the clarification Jürgen.
Quoting the note for partners with the same question:
21. Question:
You create a purchase order item assigned to an account with reference to a purchase requisition. For this purchase order item, you create a second schedule line with reference to another purchase requisition. The system ignores the account assignment data of the purchase requisition for the second schedule line.
Solution:
This is the standard system design. The system does not generate multiple account assignment in the purchase order item, even if the two referenced purchase requisitions are assigned to different CO objects.
Refer to Note 47150 for the old transactions and to Notes 422609 and 771045 for the EnjoySAP transactions.