If you're content on running multiple computers when dual boxing, ignore the rest of this post. I'm addressing the "dual boxing on one computer" issue of the launcher preventing it.
One of the best things you can do for dual boxing is to write your own launcher app. If you can get a port 80 sniffer (I use Fiddler) you'll see the HTTP and SOAP calls the launcher is making to perform the various parts of logging you in. You'll also have to make it trust the sniffer's https certificates in order for the launcher to work while it's running. For somebody with .NET services experience and the right software tools (all free), this can easily be done in a single day.
The only reason the launcher prevents multiple client launches is to protect your data files from getting trashed by 2 launchers attempting to update them at once. If you write your own launcher and just skip the patching checks, the game server will boot your client with a "wrong version" message. Just fire up the real launcher to patch and you're golden.
Once you get your own launcher running, you can start additional game clients on 1 computer any time you wish. No fancy batch files, no simultaneous startup requirements, etc. Provided you trust yourself, you can even make a command line version that auto-logs you in from a desktop shortcut and just launches the game directly.
Not that any of this is supported or endorsed, but it's theoretically possible...