You will need to connect your two batteries in series first by connecting one positive wire from each battery to one negative wire from the other. Then you will need to attach a wire between the positive terminal of your first battery and the negative terminal of your second, and then another wire from that second battery’s negative terminal to one side of the solar charger. Finally connect the other end of that solar charger’s positive side to one of your batteries, with the other end of that same battery attached to the other side of your charger.