OK, so I routed Kontakt to Outputs 5/6 on Focusrite, then ran two patch cables from Focusrite Out 5/6 to Focusrite In 3/4
Now if I select inputs 3/4 in RB I can hear/record Kontakt in real time, which is what I was after, so *solved*.
Kontakt, KompleteKontrol, Reaktor 6, etc all seem OK like this ... having to use a physical cable to route stuff thru the 18i20 did seem a little 'klunky', but it works
Just reporting back in case it helps someone else

/I really thought I would be able to do this inside RB, but didn't find a way, so I found a workaround

I do not work here, but the benefits are still awesome
Make your sound your own!