A 58-year-old man with chronic obstructive pulmonary disease (COPD) has persistent exertional dyspnoea and one moderate exacerbation in the past year despite adherence to once-daily long-acting muscarinic antagonist (LAMA) monotherapy. He continues to smoke 10 cigarettes per day. According to recent Canadian Thoracic Society COPD guidelines, which pharmacologic change is most appropriate?