Hi,
You can achieve it in standard configuration itself, with different shipping point for both the plants. Go to the path: SPRO - Logistics Execution - Shipping - Shipping Point and Goods Receiving Point Determination - Assign Goods Receiving Points for Inbound Deliveries. Here, assign the different shipping points for both the plants. Test the scenario again and check the delivery split.
In addition, refer the note: 546668 - FAQ: Delivery split when creating deliveries and 166397 - Delivery split according to customer field ZUKRL
Regards,
AKPT