These days I'm carrying two Leica's on my travels. I shoot with a Leica M(240) and a Leica M10. Shooting with two cameras allows me to leave the 50mm on one camera and the 35mm on the other. I seem to have the wrong lens on all the time. Back in the days of film I used to shoot with a black M6 loaded with black and white film and a chrome M6 loaded with Kodachrome. Shooting with two cameras has always been my operating procedure.
With digital the need for spare batteries is a must have. Unfortunately the Leica's take two different size batteries. Since the batteries always seem to migrate to the bottom of my bag and unless you check them close when changing batteries it is easy to try to put the wrong battery in a camera. I have found a battery pouch that solves my problem. It is called the Reload Battery Pouch by Tenba. I think they are designed for the standard size batteries that Nikon and Canon use but they are perfect for the M series Leica's. I have purchased two pouches and use one for the M(240) and one for the M10. The batteries fit perfectly. The are made of nylon and have velcro on the front to keep the cover closed. On the back there is a velcro strap to attach the pouch to a belt or the camera bag strap. There are also "D" rings for attaching to an optional lanyard. The Reload battery pouches come in single and double battery sizes. Nice and sturdy workmanship and they are only about $12 each so the don't break the bank. As I was searching for a case for my batteries I came across companies that made similar cases made out of leather. Kinda nice looking but no way was I going to pay $75 for leather.
I have color coded the pouches for easier identification and that makes choosing the correct pouch when I want to travel light with just a camera and batteries easier. I highly recommend these little pouches.