Twobox toolkit has been working fine for me. Not only has it been fine for notifying me when follow is broken bit it also relays whispers from my slave to my main. You may try to download a different version or make sure you are downloading the most up to date and from a reliable source.

I think I downloaded it from here http://www.wowinterface.com/download...oxToolkit.html

Now I haven't tryed the newest version 1.57, I am still using 1.55 and it works fine. The only annoying thing is it also says follow is broken when you tell the slaves to follow.

Other than that you may want to use the other addon mentioned above.