I have had someone else hold the line and another person walk next to him and "lead" him since he thinks its a lead anyway. He will walk, trot, and canter with someone next to him but when we thought he finally understood and took the person away he just kept trying to go to the middle. He likes people. A lot.
Its not absolutely necessary that he lunge on a line, I can free lunge him if need be and he doesn't really ever need to be lunged at a show, it'd just be nice to know I could.
Please excuse me if I sound really dumb about this I really don't know the art of teaching a horse to lunge :)