Heat a grill to low.
Coat both sides of the tri-tips with half of the seasoning mixture, rubbing it in as you would a dry rub. Let rest for 30 minutes at room temperature. Place the tri-tips over a low temperature fire, 1 with fat side up, and the other with fat side down. Turn as the first side gets crispy, approximately 6 to 8 minutes. Be careful of flare-ups, as the dripping fat will fuel the fire. Turn the tri-tips before the heat pushes juices out the top, and continue to turn using this timing method throughout the cooking process. After turning, baste with sauce and season lightly, 4 times per side. Continue turning until the tri-tips are cooked to your liking. Remove from fire and let rest for 10 minutes before cutting into 1/2-inch slices against the grain.
Mix together all ingredients in a small bowl.