Not sure about RekordBox DJ settings, but you might be able to set it to output the master to your Focusrite output and keep your cue signal going to your controller headphone.
Another option could be to take a stereo input on the Focusrite, connect your controller output to itand use direct monitoring on the Focusrite to make the signal go straight to your monitors (you don’t say which model Focusrite, so not sure what options you have there).
Final option is to get a desktop speaker management system. Usually a big volume knob, couple of buttons to push. They are usually setup to send multiple inputs, for example your Focusrite output and your controller output, to multiple outputs, for example near- and midfield monitors. Not a cheap option generally, although if you shop you might be able to find some basic input switching boxes.
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