Preheat a grill to low and medium: On a gas grill, turn the center burner(s) to low and the side burners to medium; on a charcoal grill, bank the coals to the sides with a few coals in the center.
Combine the cheddar, pepper jack, bacon, jalapeños and scallions in a medium bowl.
Slice the bread with a serrated knife in 1-inch intervals, cutting almost all the way through. Rotate the bread 90 degrees and repeat to create a crosshatch pattern. Set the bread on 2 large stacked sheets of heavy-duty foil.
Gently spread the bread open and brush between the cuts with 4 tablespoons melted butter. Stuff the cheese mixture into the spaces. Press the bread back together and brush the outside with the remaining 2 tablespoons melted butter; wrap in the foil.
Once the grill registers 350˚ F to 375˚ F, set the bread in the center of the grill over low heat, cover and cook until warmed through, about 15 minutes. Pull back the foil to expose the surface of the bread (do not remove). Continue to grill, covered, until the top of the bread is crisp and the cheese melts, 10 to 15 minutes. Serve warm.
When you cut the crosshatch pattern in the bread, be sure not to cut all the way through!