How to connect two 12 volt batteries together
There are a few ways to connect batteries together, really depends on what voltage or capacity your requirements are. Drilling right down to the basics, In a 12 volt configuration where more capacity is needed you would connect two batteries (or more) in parallel. For a 24 volt configuration you would just connect two 12 volt batteries in series to achieve the required voltage. Either of these can be done easily by connecting the terminals in the correct way.
How to connect 2 x 12 volt batteries in parallel
Using a jumper cable, connect between the negative terminals of both batteries. Using another jumper cable connect between the positive terminals of both batteries.
To use the parallel connection on an appliance, connect the positive and negative leads of the appliance you are using to the corresponding terminals on ONE of the batteries.
You can use either battery, but make sure you connect both leads to the same battery. Your appliance will now have use of the combined capacity of the two batteries.