You may connect your SLA battery in series or parallel.

Connecting your SLA batteries in series will generate a higher voltage. For example, connecting two 12 Volt 18Ah batteries in series will provide you with an output of 24 Volts and 18Ah capacity.

Connecting your SLA batteries in parallel will generate a higher capacity. For example, connecting two 12 Volt 18Ah batteries in parallel will provide you with an output of 12 Volts and 36Ah capacity.