Divide one bag of potatoes into (2) 9×13″ baking dishes. Season with salt, pepper & fajita seasoning. Mix the cheeses together and sprinkle half of the mixture over the potatoes. Repeat with the remaining potatoes and cheese.
Spread the onions over the cheese mixture and then sprinkle everything with the crumbled bacon. Whisk together the eggs, milk, salt, pepper & fajita seasoning, and pour the egg mixture evenly over the potato mixture. Cover and refrigerate overnight (or 7-8 hours).
When ready to bake, preheat the oven to 350. Bake, uncovered, for 40-45 minutes or until the cheese is bubbly but not brown. It could take up to an hour. Allow to stand for 15 minutes before cutting. Serve with salsa and a side of fresh fruit.