Stir together cold mashed potatoes with Monterey Jack and cheddar cheeses and diced jalapenos. Scoop 48 tablespoonfuls of potato mixture and place on parchment- or wax paper-lined baking sheet. Refrigerate for 15-20 minutes.
In bowl of food processor, pulse tortilla chips to fine crumbs, or place chips in large zip-top bag and smash using rolling pin. Pour crumbs into shallow bowl.
Beat eggs and pour into separate shallow bowl. Set aside. In fryer or Dutch oven, heat oil to 350 degrees.
Remove mashed potato scoops from refrigerator and roll into balls. Roll each mashed potato ball in flour, tapping off excess. Once potato balls are coated in flour, dip each into egg, allowing excess to drip off, then into tortilla chip crumbs, making sure to coat evenly.
Set on clean baking sheet. Discard excess flour, egg, and tortilla chip crumbs.
In hot oil, fry small batches of mashed potato poppers until golden brown, for 30-40 seconds. Remove from fryer and place on paper towel to drain excess oil. Repeat until all poppers are cooked. Serve hot with sour cream, if desired.
Note: To reheat poppers, bake in oven for 6-8 minutes at 350 degrees.