The AnyCubic Kossel comes with a single extruder and I need two. Rather than just add another I decided to replace it with two extruders of my own designed (which maximize filament support). I already use a version of this extruder on my Prusa.

The extruder is design to use a pair of Bondtech drivegears which are great for moving the filament effectively but rather pricy at $50 per extruder (there really must be something as good for less money ... anyone?)

The two extruders (plus associated idlers) were printed in Rigid Ink White ABS, assembled with cheap steppers from Amazon, and mounted on the rails in place of the old extruder.

The E-step value for these extruders was 136.2 but obviously your milage may vary.

The TriGorilla board which comes with the Kossel (and unfortunately is no longer produced) supports two extruders, so to get both working was just a matter of reconfirming the AnyCubic version of Marlin to support both.