No idea about addons, but essentially you have to main options. 1)Make an entirely new installation for WoW that is just used for multi-boxing (you can copy the original folder), then everything you setup for multi-boxing is separate from your main solo install. 2)Have a careful setup, so that any action you take can by using macros apply to your focus (follower), your target (main) or your target (solo). You do get spammed with Unknown Unit messages (and even those can be switched off).
Currently I'm working on a perfect ;) solo/dual macro layout, and for now its working absolutely great, my main is effectivly playing solo, and my follower is assisting him all the time, its completely seamless... now how well that would scale if I added two more Hunters and a Priest/Druid/etc I can't really say but I can't believe it would change much, especially if the Healer type was a dedicated healer type. I'd need more keys would be my only real issue.