There are 4 moving head "fixture type numbers" and how they move depends entirely on the macros / movements programmed into the macros. Not all of the movements are mirrored - some are "chases" and others make them all do the same thing. If you truly want mirrored, my recommendation would be to assign them both to the same DMX address, and the same fixture type number, but on the fixture itself, invert the pan function.
Give it a try by assigning the fixture types to different numbers, you may find you prefer having them set individually so they do different things as part of the same program. That would necessitate having different DMX addresses.