Re: Diodes between (-) terminals on parallel batteries. Why?

It's an attempt to prevent a stronger battery from charging a weaker battery. Sometimes used when the batteries are of two different capacities, or wide age difference.
Best to always have both batteries be the same type and age. Then one doesn't need a "patch", they keep each other in balance. Your diagram doesn't show, but depending on where the load comes off, or the charger connects, there could be issues with getting full use, or full charge, out of one or the other. Get two matching batteries, and get rid of the diodes.