I wouldn't recommend Proxydroid. It's a flaky app in my experience.
The MassPlanner suggestion is good. Seriously, it's the best application for multiple account management. You do have to pay extra for the "InstaChat" plugin though, but it's worth it because you can also see your message requests. It has everything you can do on a phone (if you put in a little extra $$$ for features) except for editing captions. Don't think I've found a way to do that yet.