At work we use a pessoa or harbridge.
I use a harbridge on my 3. I don't really like side reins and I can't use a pessoa on Gypsy (driving pony) Hattie hates it and I haven't been brave enough to try it on Inka.
Mostly I don't lunge mine with anything.
Elastic Balancing Reins, by Aerborn. Great pieces of kit - long lengths of luggage elastic with sliders which go from D rings to girth. All soft, nothing to pull agaist, just creates a comfortable place to be.
If I lunge with a single rein it's purely for exercise, for work I use two reins as it gives me more control & I feel makes any extra equipment unneccessary.
I use very long side rein to get him stretching - when he came to me he was very tight through his neck and back which made him quite weak. Plan to move onto the pessoa eventually to keep topline and muscle.