This will be a single center, randomized, double-blind study consisting of two 2-week treatment periods separated by a 5-7 days washout phase without long-acting bronchodilators. Eligible patients who complete the one week screening phase will be randomized to one of two treatment sequences: 1) Formoterol --> Formoterol + Tiotropium or 2) Formoterol + Tiotropium --> Formoterol. During the treatment periods, patients will be allowed to use a short-acting beta2-adrenergic+short-acting anticholinergic as rescue medication (salbutamol+ipratropium via MDI)