Interompany Billing Clarification

Hi,

My client has scenarios in which one Op Unit (A) is performing work as a sub-contractor to another Op Unit(B).  They have multiple projects in which this occurs as Op Unit A is required to bid on work for Op Unit B.  If they are awarded work for Op Unit B then a new project is created upon award.  In reviewing the Intercompany Billing setups I've determined that in order to use Intercompany Billing I can only have one project that actually generates Intercompany Invoices - Is this correct? If so then it seems that the Inter-Project Billing would be a better