XDJ-RX with Rekordbox DJ (mixer mode)

MacBook Pro (Retina 13-inch, begin 2015)
2,7 GHz Intel Core i5
8 GB 1867 MHz DDR3
OSX EL CAPITAN v10.11.3 (already installed when I bought this MBP last week)

Situation:
Until last month I used Serato with DDJ-SX/DDJ-SX2. I recently purchased my XDJ-RX controller. Why? To have a back-up solution when the laptop crashes. (backup = USB stick)

I play 6 hours-sets with all-round music and I also play requests, so playing with USB-stick only is not an option for me.

Issues:
When I set the mixer mode to "midi control" in Performance (DJ) mode, I cannot use the "color" effects or "basic" effects on the mixer. 
When I set the mixer mode to "XDJ-RX" on the controller, the effects are working, but the output of both channel 1 and channel 2 both appear on channel 1.

In the Rekordbox DJ settings, I cannot switch to "external mixer". This option is "greyed out".

Questions:
Why is the "external mixer" selection greyed out in "performance/DJ" mode? Will it be possible to change to this mixer mode in future?

Or is there an other option to use the mixer effects in performance mode?