Official comment
Love the setup - looks like you keep yourself busy during the mix. :)
Two identical sub-controllers can't be used at the same time by default.
You may be able to do it with a MIDI translator (like Bome, following the instructions here to remap one of the units to another channel), however I haven't tested this myself and I'm not sure if rekordbox would see that as a second device or two copies of the same.
