BuddyPress.org

You could use the following setup:

– Leave Atahualpa without modifications, or with subtle modifications like the code from Andy.

– Modify the bp-sn-parent style.css to make it a child of Atahualpa. Remove files present in Atahualpa from bp-sn-parent ( in particular header.php and footer.php, you decide if you want to remove single.php, index.php, etc ).

Now choose the bp-sn-parent as your theme. This should let you have a very basic integration of both themes.

Probably you will have problems with some pages, specially group pages. Applythe patch in this page to solve the problem: https://trac.buddypress.org/ticket/997