Combine marinade in a small bowl. Place tri tip roasts in a resealable plastic bag. You might need to use two. If so, divide marinade in half and distribute accordingly. Pour marinade over roasts making sure that all surfaces are well coated. Seal bags and place into refrigerator for 6-12 hours.
Preheat grill for medium heat.
Remove roasts from marinade and place onto grill. Take remaining marinade and pour gently over top of roasts. This can be messy, so if you'd rather brush the marinade on, then do so.
Cook tri tip for 1 hour or until it reaches 145 degrees F., or for medium-rare, 135 degrees F. Watch for flare-ups and adjust temps accordingly, or simply move to a cooler part of the grill until it is contained.
Once the roasts have cooked through, remove from grill and place onto a large cutting board. Loosely tent with aluminum foil and let the meat rest for 10 minutes. Slice and serve. The leftovers are great in sandwiches!