Comentário oficial
In most cases where the controller has a deck selector to toggle between 1/3 (for example), the hardware itself switches the output signal for all buttons, knobs, faders, etc. to send a different set of MIDI signals. Unfortunately the DDJ-400 is not designed with a second deck selector, and as such, there is no ability to send a second "layer" of MIDI messages.
You could achieve this yourself using a MIDI remapper, but it's non-standard and you could run into other caveats.
Sorry!