I've rarely had a helper when training, so haven't tried PP's suggestion. There's no problem with it, so long as you've got a helper that is in tune with you regarding your cues & timing of reinforcement.
Lunging for me is about teaching the horse to respond to your direction - enhancing communication & leadership - at a distance. Therefore, I treat it progressively from yielding at close quarters.
I first teach the horse to yield to physical pressure, from my fingertips or a stick/whip used as extension of my arm. They learn to go forward, back, sideways, yield HQ or FH, depending on where I direct the pressure to their body or the halter.
Then I teach them to yield to implied pressure - bodylanguage, a waving rope or stick, etc. Standing a couple of steps away, I start with bodylanguage(I always start with bodylanguage for everything, because this is what I want them ultimately responding to). If/when they don't respond to my focussed look, I point at the body part I want yielded & use rhythmic pressure with hand, stick, rope, to 'pulse' the energy at the horse. I get gradually closer until I'm actually tapping on the horse. Usually they move at that stage, if not before.
As with anything you're teaching, the *instant* they respond, negatively reinforce this by dropping all pressure(including body energy). I also like to use positive reinforcement(rewards) to enhance the learning & speed of training, not to mention their willing attitude. With repetition they will learn to respond to your early cues and avoid your 'bigger' ones.
Once you've taught a horse to yield to implied pressure, then it's just a matter of gradually increasing the distance and doing less moving yourself during the process, until it becomes 'lunging'.