Hi Johan,
I think in the end in my case the workaround was indeed that I used default values. So you still can manually click (+) to create the entity B (filled with defaults) if you want to, and otherwise I would use the defaults directly from the code.
In a current project I am considering keeping A and B in the same entity. And then using pages to “separate them” visually. In that case it looks like you have two entities with their own separated views, just under different tabs at the top.This allows a visually separated editor, but still everything is created at once. The downside is that this means a bit more book-keeping for the developer to make sure the right views only visible under the right page.