Here's the approach I would take:
1. Query User for your friends and materialize the results.
2. Query User for your followers and materialize the results.
3. For each of your friends, query User for their followers and materialize those followers.
4. When you get the followers for each friend, as specified in step #3, use the Intersect operator to get a list of common users.
No problem. It's good to see how people are using LINQ to Twitter, which helps me find and fix problems that people are having.