I don’t see anything in the manual.
Nothing in Instinct manual, but https://www8.garmin.com/manuals/webhelp/instincttactical/EN-US/GUID-5C6D0313-6EF4-4B8B-AF2C-937FC202AB76.html in the Instinct Tactical manual.
From https://www.facebook.com/GarminFenix5/posts/d41d8cd9/680446258822392/ :
DUAL GRID: gives you regular GPS coordinates in standard + in your selected in settings (for example, MGRS) grid, useful if you use some different maps at once and need to see your coordinates in both
