To run 2 versions on one computer you need: 2 subscriptions and 2 registered copies of wow (one for each account). You dont need to install wow twice, and can run both instances from the same folder.

If you are only running 2 accounts i dont think you need to worry about keyclone in the beginning, you could just keep both versions in windowed mode and alt-tab between them. Its a nice program though, so if you have the $20 to spare it can be quite fun to tinker around with.