As Mike said above -
The POS of the solar panel goes to one of the POS terminals on one of your batteries...
The NEG wire of the solar panel goes to the NEG terminal of the OTHER battery - this allows for an 'even' charge to both batteries & tends to even out the small resistance of the connections...
If your solar panel does not use a charge controller, make sure you have a blocking diode in the wiring circuit between the panel and the battery - this will prevent any discharging of the batteries when there is no sunlight - IE, at night!
Ray & Pat; Morada, CA