Help me think this through.  I've got two different databases (Student and Inventory), and therefore, I've got two DALs and two BLLs.  In the business layer for the Student database, I want to move some inventory around.  If I make calls from Student.BLL to Inventory.BLL, I've got to reference Inventory.BLL in my Student.BLL.  This is fine until I need to do something to student data from inside the Inventory.BLL, because then I've got to reference my Student.BLL from inside of Inventory.BLL and I've created a circular reference for builds.
What's the best approach to this?