Trim the tenderloin of all of the fat and silverskin.
Lay tenderloins on cutting board, flat side (side opposite where silverskin was) up. Using a dinner fork, scrape flat side lengthwise from end to end 5 times, until surface is completely scored. Repeat with second tenderloin.
Disolve 3 T salt in 1-1/2 quarts cold water in a large container. Submerge tenderloins in brine and let stand at room temperature for 1 hour.
Remove tenderloins from brine and pat completely dry with paper towels. Lay 1 tenderloin, scraped side up, on cutting board and lay second tenderloin, scraped side down, on top so that thick ends of 1 tenderloin matches up with thin end of the other. Tie the tenderloins together with kitchen twine, spacing evenly apart.
Brush roast with vegetable oil and season with pepper. Transfer 1/3 cup glaze to bowl for grilling, reserving remaining glaze for serving.
Clean and oil cooking grate. Heat all the burners to high, cover, and heat grill until hot, about 15 minutes. leave primary burner on high and turn off other burners.
Place roast on cooler side of grill, cover and cook until meat registers 115 degrees, 22 - 28 minutes, flipping and rotating halfway through cooking.
Slide roast to hotter part of grill and cook until lightly browned on all sides, 4 - 6 minutes. Brush top of roast with amount 1 tablespoon glaze and grill, glaze side down, until glaze begins to char, 2 - 3 minutes; repeat glazing and grilling with remaining 3 sides of roast, until meat registers 140 degrees.
Transfer roast to carving board, tent loosley with foil, and let rest for 10 minutes. Carefully remove twine and slice roast into 1/2" thick slices. Serve with remaining glaze.
Glaze: Heat oil is small saucepan over medium heat until shimmering. Add garlic, ginger, and pepper flakes; cook until fragrant, about 30 minutes. Whisk in hoisin and soy sauce until smooth. Remove pan from heat and stir in vinegar. Sauce can be done in advance.