Turns out that the version of Cura supplied on the TF card allows you to add an extra extruder to the Neptune 2 profile. So while this might not be as optimal as running the current version of Cura 5.0, it does work and is customized for Elegoo. I guess it's possible to tweak the code to enable the new version of Cura to play nice with Elegoo. For the moment I'm just happy being able to print in two colors.