Multiple Commodity Codes to Same BU.OBJ account in Requisition Self Service

I keep getting errors when trying to assign a commodity code to a BU.OBJ account that I have assigned to another commodity code as well.  Can you only have one commodity code to a BU.OBJ combination?  We wanted to utilize a 3rd level of the commodity structure to get more defined reporting on our purchasing, but I cannot get the system to accept multiple commodity codes to the same object account.

Theresa