Just down loaded 3.1 and I created an "other" printer for my T Rex 18 Monstr3D Printer. My printer will have 3 extruders so I set the "other" up with the 3 extruders and the 450x450x450 build area. Tried to use "Dual Extrustion Wizard", but it would not recognize that the "other" printer had 3 extruders. Dual extrusion did work with one of the pre configured 3 extruder printers.
Why don't you just create 2 processes, and then set both of them to the "all extruders" option? Then it's as simple as using the "Select Models" button to choose which models each profile applies to and choosing which extruder you want to use for each process.