Yoast will index the content associated with the FAQ post but will not include that on the page/post where you have created the relationship to that other post because the content is not part of that post (in your example the about us page).
To do this you would need to copy the question and answer to either the content of the about us page or fields associated with the about us page.
However, this indexing that you speak of is only an admin indexing and it does not have any impact on search engines. But if you want it for the admin report for the page then the only way to do it is to copy the content.
