If you connect two identical batteries in series, the so called Cold Cranking Amps (CCA) remains the SAME it does NOT double, although the voltage doubles, i.e. two sixes in series now = 12 volts. If each were 1000 CCA, two in series still equals only 1000 CCA...........
If you connect two identical batteries in parallel, the voltage remains the same yet the available amperage does double in that case.
In BOTH theories above, still all remains subject to internal resistances and ohms law, of course.