How do I create a transparent stroke above shapes and that stroke affecting the underlying shape?

Ok, so maybe I'm asking too much. I've seen how to achieve a knockout group stroke transparency, but I was wondering how to make the underlying shape curve to the shape of the above shape stroke

I'm here:

And I'd like to achieve the following result without having to divide the shape or anything more

This will make my life easier in the following months if this can be achieved.

Thank you in advance 🙂