mutual-followers is a tool that allows you to discover the mutual friends of twitter accounts.
I personally use mutual-followers to discover influential people on software engineering by inspecting the mutual friends of reputable people.
Currently the only method to install this is to clone the repository and run pip install on requirements file.
Note that you need twitter consumer and secret keys for mutual-followers to work.
Please take a look at https://stackoverflow.com/questions/1808855/getting-new-twitter-api-consumer-and-secret-keys to get your consumer keys.
run this with arguments --consumer_key <> --consumer_secret <> --access_token <> --access_token_secret <> --users dhh,jasonfried
and you will find out who @dhh and @jasonfried is following mutually.