The functionality you mentionad is also implicitly available in ADOxx.
The issue can be solved by changing the event handlers for the events "CreateRelationInstance", "BeforeCreateRelationInstance" and "DeleteRelationInstance".
I have further developed the application library for Implementation 2b into version 1.2 and added the classes AA and BB derived from A and B, respectively. Please see attachment
CIDOC_MultipleInheritance_Implementation-2b_1.2.abl .
I have also changed the implementation of the event handlers in the file "ExternalCoupling.asc" to allow the connectors between A and BB, AA and B and AA and BB, respectively. Please see attachment
ExternalCoupling.asc .