Thanks for you answer.

You should find useful abstractions based on how they interact with each other, not by who develops them.

I agree, this was not a good example. But as I mentioned these modules are logically separated, so I wanted to introduce the abstraction in the class design.

Looks like delegation would make your life easier here.

Thanks for that tip and also to John M. Dlugosz for mentioning delegation, too. I have not looked into this before, will definitely do that now.