2 X 8V batteries in series would be 16V  (actually a little more that as wet cell batterys are slightly more than 2V per cell)so you would have to adjust your regulator to put out more than the actual voltage or they would never fully charge. The old  thing of cranking longer applies if you have the same grade of batteries. The cranking amps (32 deg) and cold cranking amp (0 deg) are an instantaneous rating, when buying batteries you need to look at the number of plates and the minutes of reserve capacity, these ratings will give you the correct info you need to compare batterys. One high end 12V will out last two cheapie 6V in series.