Preheat a grill on high. Place a grill pan or cast iron pan on the grill to heat.
Season the chicken thighs and flank steak liberally with 2 T of the fajita seasoning, using more if necessary.
Add the peppers and onion to the preheated pan. Season with the remaining 1 T fajita seasoning and sprinkle with 1 T olive oil. Cook, tossing occasionally, until softened, golden and slightly charred.
Meanwhile, add the chicken and steak to the grill; turning the chicken after 4 minutes and the steak after 6. Cook the other side for an additional 3 minutes and 5 minutes, respectively. Remove from the grill and allow to rest for 5-10 minutes. Slice thinly and add to the pan with the peppers and onions. Toss together and serve very hot, straight from the grill.