Let's get our terms correct; the Battery has 3 cells in series for a total of 6V, nominal. We take two BATTERIES, in series, a total of 6 cells to make 12 volts. This is EXACTLY the same arrangement as any other Pb-acid based battery we assemble. Doesn't matter how we make the 12V, 6x2V individual cells, 3x4V batteries or 2x6V batteries. The 12V battery runs the same risk of shorted cells, unbalanced charge and total destruction. Using the same capacity AHr for each cell or battery in the assembly is critical to the whole longevity of the battery, but the assembly, up to 12V is really for the convenience of using of-the-shelf chargers.

I'll date myself here, but I remember assembling one 12V battery and one 2V cell to get 14V to put in the glider. The old radios were useless if the voltage dropped below 12 volts and just keying the mic would drop the voltage too low if you used a 12V setup. It was almost impossible to find a 2V Pb-acid charger in the late 60's.