Heat grill. Cut four 18x12-inch pieces of heavy-duty foil. Make 6 or 7 crosswise cuts in each potato, cutting to but not through bottom. Spread margarine between slices.
Place each potato on piece of foil. Place half onion slice in each cut. Sprinkle potatoes with salt, garlic powder and pepper. Add 1 teaspoon water to each potato. Wrap each packet securely using double-fold seals, allowing room for heat expansion.
When ready to grill, place packets, seam side up, on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium-high coals. Cook 35 to 40 minutes or until potatoes are tender.
To serve, open each packet carefully to allow steam to escape. Sprinkle each potato with Cheddar and Parmesan cheese. Close packets until cheese is melted.