Make sure you wire them parallel so that you maintain 12v, positive to positive, negative to negative. If you wire in series, the voltage will double to 24v, positive to negative. You only want to wire in series if you were using 2 6v batteries to give you 12v, but if you're using 2 12v batteries you want to maintain 12v so therefore wire in parallel. Also make sure to use the correct gauge wiring depending on you DC amp draw.
Also, always best to use 2 fresh batteries as an older battery could bring down a newer battery once they are connected. For example, batteries will equalize when wired parallel. So if one battery was 12.4v and the other was 12.8v, it would measure 12.6v when they are combined. Usually a good fully charged 12v battery will measure 12.7 - 13v with no load. Also measuring your old battery with just a volt meter won't tell true battery health, you should do a load test on your existing battery to insure it is still performing as new. Make sure you use deep cycle batteries and good luck.
Hope this does not confusing!
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