Well since he has a 12 volt motor he only needs 12 volts. Don't use the jumper in the plug to add the voltage, use both batteries in parallel. jumper the positives together in the male plug and jumper the negatives together.
As far as jumpering the two 12 volt sources IN THE PLUG here is a quote so you can see how the current divides:
Total current in a parallel circuit is the sum of the individual branch currents. This relationship in a parallel circuit is expressed as: IT = I1 + I2 + I3… Whenever more resistances are connected in parallel, they have the effect of reducing the overall circuit resistance.