Dear,
You can define these products as co-products / joint product . You need to maintain those source structure, equivalence numbers etc. Costing will be determine on the base of source structure and equivalence number
For more details, refer to following link:
http://help.sap.com/erp2005_ehp_03/helpdata/EN/90/ba620e446711d189420000e829fbbd/frameset.htm
http://scn.sap.com/thread/1411970
Regards,
R.Brahmankar