Two of the WDB1220LSM dual battery chargers can be connected in parallel to provide double the current (2 x 20A = 40A) for much faster charging. This can be easily be accomplished by connecting the "CAR" inputs together and also the "AUX" outputs - see diagram. The solar inputs will remain independent. All other specifications and method of operation are the same as a single unit, please check here: Single Dual Battery Charger

Connection diagram in detail: 40A Setup Diagram

Note: not all lead-acid batteries are able to be charged at a rate of 40A - refer to the battery datasheet. However, lead-acid batteries of around 150Ah or more usually will be adequate. Alternatively, two or more 100Ah batteries can be connected together to act as a bigger capacity battery that can handle the high charge rate. Lithium batteries can usually be charged at a maximum rate of 0.5C to 2C. For example, a 100Ah lithium able to charge at 0.5C gives: 0.5 x 100 = 50A maximum allowed charge rate. More information on batteries and charging of batteries can be found here: batteryuniversity.com