Hi,
You can define the BOM appliction as no original selection in OS30 and assign it to Order type in OPL8 so that there will be no selection of BOM while creating order and maintain the by product and co product in Order in component tab with negative sign and select the Co product check box before making an entry. also in that case you need to give settlement rule in every order.
Check and revert
Regards,
Anupam Sharma