Hi Jasmin,
You can control this on a customer-by-customer basis on the Account master data (under the Sales Data section):
This checkbox is on by default, clearing it will ensure that Sales Orders created for the Account use the "Multiple Deliveries" option.
Note that "Complete Delivery" in any case only works if all line items on a Sales Order are sourced from the same Supplier. In all other cases, "Complete Delivery" will block the Sales Order due to the inability to calculate a delivery date (ATP check).
Hope this helps,
Eelco