Kickstarter idea - one hand lens cap swapping
Another way to handle the problem is with a "double ended" back cap. This allows you to remove the lens from the camera, place it in the cap, and put on the alternate lens. You never have to fumble with caps again.
This shows an official Leica version of the double-ended cap. I have made similar caps for Nikon and Canon by gluing two caps together with a plastic solvent. Pretty easy to do....