Make sure the batteries you're connecting are of the same voltage, capacity, and battery type. Failing to do so can result in shortened battery life.
Use jumper wire to connect: The positive terminal on Battery 1 to positive terminal on Battery 2. The negative terminal on Battery 1 to the negative terminal on battery 2. Then connect the positive and negative wires to the corresponding terminals on your application.
A parallel connection... doubles the Ah capacity of the batteries while keeping the voltage the same. For example, wiring two EnerSys NP5-12 (12V, 5Ah) SLA batteries together will retain a voltage output of 12v, but it will double the capacity to 10Ah. This wiring configuration is often used in scooters, backup power supplies, and laptop batteries.