Given that they are identical units I'm going to say you need a crossover cable.
Page 8 of the manual - http://docs.pioneer.eu/Manuals/S_DJ08_DRE1056_manual/
"In the event that the desired installation requires a longer Link cable than the one provided, purchase a commercially available MINI DIN 8a extension cable and connect it to the furnished Link cable.
WARNING
Do not use a commercially available MINI DIN 8a extension cable alone; always connect it to the Link cable furnished as an accessory with this unit, since malfunctions, fire, or electric shock could result if the furnished cable is not used"