As far as I know, you can't do this in a single API call. You can,
however, call /friends/ids and /followers/ids and use the intersection
of the two arrays to find what you're looking for.
Relevant API documentation:
As we can retrieve friends and followers of the user using the twitter
rest api, can we also retrieve
users who are both friends and followers of the user at the same time
using the twitter rest api?
Twitter developer documentation and resources: http://dev.twitter.com/doc
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Issues/Enhancements Tracker: http://code.google.com/p/twitter-api/issues/list
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