Preheat the oven to 425°F. Line a large sheet pan with parchment paper and set aside.
In a small bowl, whisk together the olive oil, ranch seasoning mix, and black pepper.
In a large bowl, toss the diced potatoes with half of the ranch mixture until evenly coated. Spread the potatoes in a single layer on the prepared sheet pan.
Bake for 60–70 minutes, flipping halfway through, until golden and crispy.
While the potatoes bake, marinate the chicken. Add the diced chicken to a large resealable plastic bag or bowl. Pour in the remaining ranch mixture, seal or cover, and toss to coat. Refrigerate until ready to use.
When the potatoes are done, reduce the oven temperature to 400°F. Lightly spray a 9×13-inch baking dish with nonstick spray.
Transfer the roasted potatoes to the baking dish and spread them out evenly. Top with the marinated chicken pieces.
Bake for 18–20 minutes, stirring once halfway through, or until the chicken is cooked through and reaches an internal temperature of 165°F.
Remove from the oven. Sprinkle the cheddar, Monterey Jack, and crumbled bacon evenly over the top.
Return to the oven for 5–10 minutes, or until the cheese is melted and bubbly.
Garnish with chopped chives and a drizzle of ranch dressing. Serve warm.