It is similar to Facebook's Friends in Common feature
Twitter has started testing new 'You Both Follow' feature which might be rolled out in the near future. This feature allows users to see common followers they share with any Twitter user. Nick Kallen, software engineer at Twitter, sent a Twitter update stating that You Both Follow feature is being tested with 10 percent of Twitter users.
The new You Both Follow feature is pretty much like checking out common friends. For instance, if you're following Techtree on Twitter and visit its profile page, you'll see list of users that both - you and Techtree follow on Twitter. This list would appear just above the Following tab that is full of user profile picture thumbnails.
This is just like Facebook's 'Friends in Common' feature wherein your common friends are shown just above the Friends list of that Facebook user. For Twitter, there's a service - friendorfollow.com that checks and lets you know about the users you follow and don't follow you back; the users who follow you and you don't follow back.
Looks like Twitter is slowly planning to interconnect more Twitter users. The website hasn't confirmed when the feature will be rolled out to the rest of the Twitter users.
Apparently, Marcel Molina, Twitter API engineer, sent a Twitter update stating that he has joined hands with Nick Kallen for building 'rapid prototypes'. Keep tweeting as who knows when you might get lucky to test one of these new features.