When I say two batteries in parallel, I am talking of a working battery and a freshly recharged replacement of the same voltage. They can be on line together for a while if you want, or just long enough to switch over to the new battery. The diodes ensure that no current will flow in to or out of the lower voltage battery.
What you would normally do is connect a fresh battery and turn on its disconnect switch. You would then turn off the old battery’s disconnect switch (which, since the new stack is providing the current, won’t have an arcing issue) and disconnect it for recharging. If you are going out of town for a while, you can leave both batteries connected so it will still be up when you get back.