Whisk the vegetable oil, lime zest and juice, garlic, chili powder, cumin, coriander, brown sugar, 1 teaspoon salt and a few grinds of pepper in a small bowl. Put the steak in a medium bowl. Place the chicken between 2 pieces of plastic wrap and pound on both sides with the flat side of a meat mallet or a heavy skillet until 1/2 inch thick. Transfer to a separate bowl. Divide the marinade between the steak and chicken and toss well. Refrigerate 30 minutes.
Preheat a grill to medium high. Oil the grates. Remove the steak and chicken from the marinade and season with salt. Grill, covered, 3 to 5 minutes per side for the steak and 6 to 7 minutes per side for the chicken. Transfer the steak and chicken to a cutting board and let rest 10 minutes.
Meanwhile, brush the bell peppers and onions with vegetable oil; season with salt. Cover and grill, turning, until charred, 6 to 8 minutes. Transfer to the cutting board. Wrap the tortillas in foil and warm on the grill, flipping once, 5 minutes. Slice the bell peppers and separate the onions into rings. Slice the steak and chicken. Serve the meat and vegetables with the tortillas and assorted toppings.