In the standalone Cura software itself, this feature is discussed on their forum. So for fun, I just told Cura that I have a second extruder in the Settings -> Printer -> Manager Printer area. Now in the print setup area and drilling into Support, I have Dual Extrusion as a new option that's available... which only gives me Enable Prime Tower and Enable Ooze Shield.
I have to actually click Generate Support to see more options. There appear to be several involved.
So, I've found the location on my computer where the Cura app stores settings which I make in that software: /Users/justincase/Library/Application Support/Cura/3.3/cura.cfg
In theory, one might be able to make the setting changes and then compare before/after versions of that file to see what variables Cura uses to store those. And then those same variable names are hopefully used in the Cura-as-a-plugin within OctoPrint.