Here is a simple web service where you can find out which of your twitter friends know each other by using TweetWheel. TweetWheel displays a connection wheel that comprises of all your friends. All you got to do is to hover on a user to see how many of your friends have common friends with you. You can click to see his/her twitter profile page.
However, if you have too many twitter friends, it might take a much longer period than normal.
This could be really fun! I mean, finding all our friends on Twitter
I knew something like this would come along for Twitter lol.