Default branch from ship to address

I have a common environment for many companies.  I have a common address book for all.  I have 2 companies with different lines of business that both have many branch plants.  Each of those 2 companies have their own central point of order.  I have the processing option set behind sales order entry to default the branch from the ship to address.  The customer service staff types the ship to address and the branch will default from the address book.  How can I deal with both company having a common customer and both using the same ship to address?