With TWO 12 volt batteries the hook up is done with both of the POSITIVE terminals hooked together and connected to the positive cable going to your inverter. The two negative cables are connected together and go to the chassis ground connection. Between the positive cable going to the inverter and your chassis ground you should read 12+ volts.
I should ad that with both batteries independently DISCONNECTED, each should read 12+ volts. This will insure that neither battery has shorted out or is defective.