which steps are needed to create a SR for an instance belonging to another Operating Unit

We have a problem when trying to create SR for equipment sold to other operating units in the company. Let me explain it.

We have various operating units, let´s call them OU-A and OU-B.

On OU-A we create a sales order and ship the material to OU-B (it´s one of our Operating Units and also a customer from OU-A). The instance ( INS1)  goes from OU-A to OU-B, but we can still see it and create SR´s and so on.

OU-B creates a Sales Order and ship the material to Ext-C ( external customer C). It is a customer from OU-B and has nothing to do with OU-A. From OU-A we have lost track of this instance, we cannot see it any longer and we cannot create SR´s for it.