Switching colors on same layer with dual extruder printer

I have a FlashForge Creator Pro 2, and I am trying to get better at complex prints of multiple colors.Something I would like to understand how to do, but cannot figure out is take this object:https://www.thingiverse.com/thing:1997136/and make it where the eye parts sunken in are in black and the rest is white. This would match the character from the show who wore the mask.This means the extruders would both have to contribute to the same layer when reaching the eyes, but I really cannot figure out how to specify this in any of the common 3D printing software. I have watched countless youtube videos related to customizing STL files and mixing and matching colors, but I can't figure out which software to use for this example or how to do it. I really, really would like to learn how to make this example work as it would fit others I would like to print also.Thanks for any help!