Heat half of the olive oil in a heavy skillet over medium high heat. Add the chicken in a single layer (or work in batches if needed) and cook until nicely browned and just cooked through. Let is cook on one side for at least 1-2 minutes so it browns before flipping or stirring. Set aside.
Add the remaining oil to the pan. Add the peppers and onions and sprinkle with the remaining fajita seasoning. Cook for 4-5 minutes, stirring occasionally, until they are tender-crisp and have some browned edges.