Probably just a mis-statement but for clarification: If you are trying to extend the battery life or double the available amperage hook them up in parallel; if you are trying to double the voltage hook them up in series (i.e. get 24 volts from two 12 volt batteries.

Parallel is when you connect both positives and both negatives together; series is when you connect the positive terminal on one battery to the negative terminal on the other.

Wouldn't want someone to fry their gear... ;-)

Buzzzzzfish