The DDJ series are strictly DJ controllers, meaning they rely on a connection to a computer with compatible DJ software (i.e. Rekordbox DJ) in order to function. They can also act as a standalone mixer if you wanted to plugin external hardware (CDJ/XDJ, turntables, etc.).
You could go b2b with another DJ if they had Rekordbox on their own computer. They would simply plug into the unused USB input on the controller (example: if you were using USB A, they would plug into USB B), assign the respective channel/channel input and then takeover from there.
Chances are you misread the mentions about developing the functionality of the DDJ et al. controller series with regards to being able to perform using an external drive (or via LINK Export Mode) only. They're not capable of performing such a task as that's not how they were engineered. In other words, they lack the necessary internal components to enable that type of functionality. Only the XDJ and NXS series are capable of standalone operation with external Rekordbox formatted drives and/or LINK Export Mode.