We prefer unenhanced pork in this recipe, but enhanced pork (injected with a salt solution) can be used.
1. FOR THE PORK: Combine salt, sugar, cumin, and chile powder in small bowl. Reserve 1/2 teaspoon spice mixture. Rub remaining spice mixture evenly over surface of both tenderloins. Transfer to large plate or rimmed baking sheet and refrigerate while preparing grill.
2A. FOR A CHARCOAL GRILL: Open bottom vent completely. Light large chimney starter filled with charcoal briquettes (6 quarts). When top coals are partially covered with ash, pour evenly over half of grill. Set cooking grate in place, cover, and open lid vent completely. Heat grill until hot, about 5 minutes.
2B. FOR A GAS GRILL: Turn all burners to high, cover, and heat grill until hot, about 15 minutes. Leave primary burner on high and turn off other burner(s).
3. Clean and oil cooking grate. Place tenderloins on hotter side of grill. Cover and cook, turning tenderloins every 2 minutes, until well browned on all sides, about 8 minutes.
4. FOR THE SALSA: Brush tomatillos, scallions, and garlic with 1 teaspoon oil. Move tenderloins to cooler side of grill (6 to 8 inches from heat source) and place tomatillos, scallions, and garlic on hotter side of grill. Cover and cook until tomatillos, scallions, and garlic are charred on both sides and softened, 8 to 10 minutes, and until pork registers 140 degrees, 12 to 17 minutes, turning tenderloins every 5 minutes. As vegetables and tenderloins reach desired level of doneness, transfer vegetables to plate and transfer tenderloins to carving board. Tent tenderloins with aluminum foil and let rest for 10 minutes.
5. While tenderloins rest, roughly chop scallions. Pulse scallions, tomatillos, garlic, cilantro, serranos, lime juice, reserved spice mixture, and remaining 2 tablespoons oil in food processor until mixture is roughly chopped, 4 to 6 pulses. Transfer to bowl and season with salt and extra lime juice to taste.
6. Slice tenderloins crosswise 1/2 inch thick. Serve with salsa.