Make sure both batteries are the same, equally worn and not worn to the same degree. If not, their serial or parallel connection refuse.
To connect two batteries in series, connect the negative pole of the first of them with the positive of the second. Terminals, the remaining free (positive terminal first and negative second), among themselves do not connect in any case, otherwise there is a short circuit. They remove the stress, increased in two times.
To connect two batteries in parallel, take two diodes, able for an unlimited period to sustain the load current. The negative pole of the battery connect together. The positive pole of one battery connect to the anode of the first diode and the other anode of the second diode. The cathodes of diodes connect together. Connect the load negative terminal to a connection point of cons battery positive to the connection point of the cathodes of the diodes. This design will give the same current two times longer than a single battery. But loading it twice large current is impossible, because always open only one of the diodes.
To connect batteries in parallel without diodes are not recommended as they will be discharged through each other. Do so only with the full confidence that they are equally loaded and worn. But with this design you can remove two times more current.
Before to charge the battery, disassemble design. Charge them individually. This will dramatically reduce battery life. If the batteries are connected in parallel using diodes, charge them without dismantling the structure, moreover, is not impossible.
After charging is complete, assemble the design of the back and connect it to the load. Continue operation of the battery.