Site architecture - consistent paths, breadcrumb and menu
I think instead of using Article content type, you should use Book content type.
so you could create a book page named a-page, then a page named b-page and provide it a-page as parent in options below.
Now start adding article-1 and article-2 as child of b-page.

So when you view these pages article-1 or 2 the breadcrumb and will provide the url as site.com/a-page/b-page/article-1

Correct me if i misunderstood the question and do tick right if the answer is right.