This adapter is specifically designed to allow two batteries with Deans connectors to be hooked together in parallel.  The two batteries MUST be identical in both voltage (the number of cells), i.e. 2S, 6S, etc., and in capacity (mAh rating), i.e. 2000mAh, 3500mAh, etc.  Connecting two identical batteries together in parallel does not increase the voltage, but it doubles the capactity available.  This connector allows two smaller batteries of the correct voltage to be used in place of a larger capacity battery for larger ESCs/motors.  For example, two 3S 11.1 volt 2000mAh batteries connected together in parallel will produce the equivalent of a 3S 11.1 volt 4000mAh pack.
 
Note:  If you're unsure of which adapter you need, remember that the battery will always have a female connector, the ESC (or other device) will always have a male connector.  The gender of any electrical connector is determined by the metal (brass or nickel pins or prongs) part of the connector, NOT by the plastic shroud or housing.  The brass male connectors insert into the female connectors.  This rule is true for all electrical connectors, including those used in R/C models.