Why are batteries connected in parallel?
When making a connection in parallel, special attention is needed for the cabling used. It is of the utmost importance that all the wires from terminal to the central conductor are of the same length. A difference in lenght means a difference in resistance which results in an undercharged battery on one end of the chain and an overcharged battery on the other end.
2 batteries 12V / 120Ah in parallel become a 12V / 240Ah system.