Advanced open-angle glaucoma and chronic bronchitis

I use Spiriva (tiotropium bromide, an anticholinergic) for COPD. Since cough and secretions are no longer being controlled, I'd like to use a combination inhaler with a beta-agonist. Theoretically, this may cancel out the beta-antagonist contained in Cosopt drops (timolol) that I am using twice daily. On the other hand, they may be working at different receptors; moreover, the inhaler should be working only topically, whereas there may be some systemic absorption of the eyedrops. Has anyone had a similar dilemma? Thanks.