For what it is worth ... Rule of Thumb: Paralell [2 reds & 2 blacks] - Double current, same voltage out ... Series [black1 to load (-ve) - red1 to black2 - red2 to load (+ve) - Double voltage, same current out.
There is somewhat of a danger in running two batteries in paralell. If their capacities are not closely ballanced, they tend to discharge into one-another, setting up a voltage chase. This causes battery heat and a lowered capacity battery-set for no apparent reason. Batteries in this mode should each be fitted with Schottkey (for lower voltage loss) diodes of the relevant load current rating (Amps draw) to prevent this phenomenon. It doesn't always happen, but the risk is there!