A double-ended rear lens cap allows you to safely stack two lenses together in a bag, end-to-end, while allowing easy one-handed access to both. It solves a common problem and you can make it yourself with no tools, no skill, and almost no money.
Many camera bags (especially “messenger-style” ones) are tall and easily handle relatively long lenses, like a 70-200mm. But most lenses are quite a bit shorter than that, so a lot of space is wasted at the top of the bag. So I have always done what I imagine a lot of other people do and put one lens down in the bottom with a padded divider horizontally on top of it and a second lens on top of that. This has a number of disadvantages. First, I always seem to end up trying to hold the top lens and get to the bottom one—through the extra divider— all with one hand. Second, the horizontal divider presents a catch-22: if it is attached via velcro on two sides, it makes access that much harder; if it is only attached on one side, the top lens can worm around it and then the lenses are banging against one another. Finally, since the top lens is not very deep in the bag, it is easier for it to tumble out of the bag altogether. The other approach I have tried is to put one lens in a padded bag and put that in first and put the second lens on top of it. It works, but it is even slower to access the bottom lens.
My solution is to simply epoxy two rear lens caps together. Then the pair of lenses make a single unit that slips nicely into one spot in the bag. To access the bottom lens, grab the whole stack, pull it out, flip it end-for-end, and put it back. Now it is on the top and easy to get at. And all with one hand. Out of caution, I am careful to grab the stack in the middle so I am holding both lenses at once. This prevents one of the lenses from coming off its cap. Also, realize that removing one lens can loosen the other from the double cap, so it is good to twist everything back together afterwards.
One of my favorite little lenses is the Voigtlander 40mm f/2 Ultron pancake lens. It is so tiny that there is almost always room for it on top of something else. Also, this scheme is a nice way to bring along a teleconverter or a stack of extension tubes. Using the double-ended caps, I can get the most out of my (sadly discontinued) Think Tank Photo Urban Disguise 20 camera bag; this bag makes a nice lens carrier for walk-around photo shoots. Also, although it is certainly snug, I can get my D800 end-wise into one side of this bag with the aforementioned 40mm pancake lens mounted and use the double-ended cap to stack two other lenses in the other half. That’s a lot in a pretty small bag.
The project only takes a few minutes. I ordered a sack of rear lens caps on Ebay and bought some Gorilla Glue epoxy. You can follow along with the pictures below. Just read the instructions for the epoxy.