Chad's right... Think of them as batteries (and actually they are just fast-charging batteries if we think about it). Put two 1.5-volt batteries in series, you get 3 volts at a reduced current supply. If you put two 1.5-volt batteries in parallel, you get 1.5 volts at double the current supply. (Not trying to play engineer, and I can't recite the laws about it)
Charles