Well, my experience with two 12 volt batteries in parallel, or two 6 volts in series is that one will go bad before the other. Then you're faced with replacing both or putting a new one with the old one.
My next will be a single 4D deep cycle battery. It has more amp hours than two 27s. Sure, I'll have to mount the bigger box, and have help to pick the battery up, but life will be simpler.
https://www.apexbattery.com/12v-200a...SABEgKIGfD_BwE