Ok so this isn't the perfect solution, but it's good enough for me.
Many thanks to @Hentoneuf's post (https://forums.pioneerdj.com/hc/en-us/community/posts/206876286/comments/207866526) for pointing me in the right direction.
I've found two different configs to achieve this, I personally prefer the second one, but I'll put them both :)
You need to edit the "rekordbox3.settings" file which on Windows can be found in the "AppData\Roaming\Pioneer\rekordbox" folder. (Use shortcut %AppData% to quickly get there)
Then once that file is open, scroll down until you find the section DEVICESETUP with the audioInputDeviceName="DDJ-RZ ASIO" - it should look roughly like below:

The important settings we will modify in that block are:
OutputChannel_Master_L="0" OutputChannel_Master_R="1"
OutputChannel_Cue_L="2" OutputChannel_Cue_R="3"
As you might guess, the numbers 0 - 3 represent the different physical outputs on the RZ, and by changing these we can achieve SC.
Config 1 - Master Left Earphone, Cue Right Earphone
Make the following changes (in bold):
OutputChannel_Master_L="2" OutputChannel_Master_R="2"
OutputChannel_Cue_L="3" OutputChannel_Cue_R="3"
This will give a true Split Cue, however has the downside of *always* being like that (so you cannot get both sides playing the master if you wanted to)
Config 2 - Master both Earphone, Cue Right Earphone
I much prefer this, as when not cueing up, you can listen to the master on its own in both ears (the equivalent or taking off your headphones when in the mix usually), to do that, make the following change (in bold):
OutputChannel_Master_L="0" OutputChannel_Master_R="1"
OutputChannel_Cue_L="3" OutputChannel_Cue_R="3"
I also like this method as if the cue is too quiet (or even too loud) - you can use the Cue/Master blend knob to adjust.
And that's it - remember to also turn on the "Master Cue" (top right of the unit) but apart from that, you can just select cue channels as normal :)