Books of Business for Child Objects

Hello we are currently using books of business on the 'Opportunity' object.
We are introducing a custom object, and would like changes to the opportunity books to be replicated to the custom object.

Example:

• Opportunity 1 is assigned to Book1.
• I create a CO1 Record as a child to the Opportunity. The workflow assigns Book1 to the CO1 record
• I then update Opportunity to be assigned Book2 also.

Without using integration events and webservices, if there a way of given users of Book2 access to the the CO1 record.

Any help would be appreciated.

Thanks