Twitter sends an email notification mentioning so-and-so-person [with their tweets, followers, etc info] is following me, the moment someone follows me.
But Twitter does not send any email after someone unfollows me.
Is it possible to know when someone unfollows me on Twitter?
No, It's part of the deal that they are not sharing your e-mail with the rest of the world.
One method would be to automatically retweet everything by the one account. A quick web search shows that there are multiple ways of accomplishing this. One way is using Yahoo! Pipes. (An example of how to do that: http://hubpages.com/hub/How-to-automatically-retweet-on-Twitter-using-Yahoo-Pipes-and-Twitterfeed-auto-RT-explained)
Best Answer
I use http://goodbyebuddy.com/. Works well, with a nice responsive UI