If you carrier 2 extra lenses in your bag this is a very practical and space saving device. It was included with some of the bags for the M series that Leica made and was also available by separately. With the Leica Lens Coupler one slot will be freed up in your bag for a small item like a smartphone or memory card holder. This could be a do it yourself project by gluing two back caps together but this is so much better and safer. No worrying about glue loosing it's grip and the caps separating. Also the coupler is much smaller so the combined lenses will not stick up to far in a shallow bag. The idea of the coupler really only works on Leica lenses because Canon or Nikon lenses are big enough already. Attaching two of these lenses together would be to heavy and awkward. A drop risk just waiting to happen. A word of caution, the Leica Lens Coupler can not be used with certain Leica and non-Leica lenses. The rear elements protrude to far from the rear and would touch the adjoining lens. Unfortunately Leica no longer produces this item but the can be found on ebay or through an internet search. It would be nice if they started producing this sweet little gadget again.
Leica Lens Coupler 14 838