Well the point I got from that post, regarding watching good mages, was not so much that you can box like they single-play, but more so that you can learn the cast sequence of spells they use which will improve your game.

There's no way you can maneuver multiple toons as well as someone can play a single toon, but you can watch the order in which they cast spells.
You can see when they frost nova or blink or ice block. You can learn which of your tools should be used in which situation.
You won't necessarily play like them, but you'll broaden the horizons of which tools are best used at which points, ending up with an improved game.