Offizieller Kommentar
Dear Sir,
Please go [Preference]>[Audio]>[Mixer Mode], and see the setting of "Mixer Mode".
If your dj controller is connected to rekordbox(PC/MAC), it may be a "Internal" automatically.
If "External", you have to switch manually between DECK1 and 2 as you said.
Also refer to the below manual.
https://rekordbox.com/_app/files/img/rekordbox5.23_lighting_operation_guide_EN.pdf
(49page, 8:Using an external mixer)
Thanks,