Offizieller Kommentar
@Diana > You need to ensure you have the drivers properly installed, then select the CDJ/XDJ ASIO driver from the device selection, and configure rekordbox for use in EXTERNAL MIXER mode.
Set your outputs for deck one to be CDJ/XDJ ASIO OUTPUT 1 & 2, then deck two for CDJ/XDJ ASIO OUTPUT 3 & 4.