Add the remaining 2 teaspoons oil and shallots to the pan and cook, stirring often, until beginning to brown, about 30 seconds. Whisk the broth mixture and add it to the pan. Bring to a simmer, scraping up any browned bits. Reduce heat to maintain a simmer; cook until the sauce is slightly reduced and thickened, 30 seconds to 2 minutes. Add the chicken; return to a simmer. Cook, turning once, until the chicken is heated through, about 1 minute. Remove from the heat and stir in orange zest.