AFAICS in the Kontakt 5 manual, things become interesting, starting from page 137 (‘Working with Output Channels’)…
Once done, in the ‘VST instruments’ window (F11), don’t forget to use the little icon which looks like an arrow going out of a box, and is right at the left of each instrument slot : this one allows you to choose which outputs/busses coming from the instrument (Kontakt 5, in this case) will be actually used in Cubase.