Dear Sank ,
there is only one option you can assign partner function in your Delivery Header and item level
As per my knowledge you can achieve this by two ways .
1. Create one new Delivery document and assign the new Delivery partner determination procedure for other units and existing Delivery document you can use existing Delivery partner determination procedure for one business unit .
2. this you can achieve through user exit , write a logic in MV50AFZ1 that according to my business unit this partner function will trigger to my delivery document both header and item level .
note - wait for other member's reply for any valuable solution you may get for this issue.
Regards,
C.B Reddy.