Cook bacon in a skillet until crisp. Drain on paper towels and crumble.
Stir chicken with ranch, garlic powder, and black pepper in a bowl.
Warm a large skillet over medium heat and melt half the butter.
Lay one tortilla in the skillet. Sprinkle on some cheddar and Jack, spoon over chicken, scatter bacon and green onion, then top with more cheese and a second tortilla.
Cook 2–3 minutes per side, pressing lightly, until golden and melty. Repeat with remaining tortillas, adding more butter as needed.
Rest 1 minute. Slice into wedges and serve warm with extra ranch if you like.