Assume that the two batteries are identical; connect the positive of the first battery to the negative of the second battery. The voltage output from the negative of the first to the positive of the...
You would wire them in "Parallel". That means positive post to positive post and negative to negative. Two 12 volt batteries wired this way doubles the capacity and amperage capability of the system....
12 volts nominal.
A 12 volt car lead-acid battery will measure about 12.2 at full charge and no load. Car charging systems usually keep the voltage around 14 when the car is running, though it might...