Hi
If you are planning to show sales and COGS separately for external and group customers, ideally, you should be showing inventory also separately. If that is the case, you can look at the option of having split valuation activated for material. In that case, the material no will remain same, but you will have valuation type assigned to two separate valuation classes and hence you can maintain COGS and Inventory separately for external and group customers. The below link for split valuation will be of help to you.
http://scn.sap.com/docs/DOC-41997
Thanks & Regards
Sanil Bhandari