Your batteries should have 12.6 volts when fully charged, 2.1 volts per cell X 6 cells = 12.6 volts. Have you had the batteries load tested to make sure they are maintaining the proper voltage while cranking? Too much voltage drop will keep the starter from turning properly; too much current draw from the starter will also cause problems. Are the battery cables tight and in good condition?
I AGREE, THAT IS FOR EACH BATTERY! WERE BOTH INSTALLED AT THE SAME TIME? DOES ANYONE USE A HYDROMETER ANYMORE? YOU HAVE A TOTAL OF 12 CELLS IN 2 BATTERIES, IF ANY CELL IS DEFECTIVE IT WILL AFFECT ALL OTHERS. JUST LIKE YOUR FLASHLIGHT.