I could find no recent references about the use of these 2 medicines together. There was a study that confirmed the pharmacologic compatibility of mixing these 2 medicines. The study found that Mucomyst® (acetylcysteine) and Atrovent® Inhalation Solution (ipratropium bromide) were stable in nebulizing solution within one hour of mixing.
As you have stated Atrovent® Inhalation Solution (ipratropium bromide) is likely to cause dryness and this may alter the thickness of the mucus. I would be inclined to use a beta-2 agonist with the Mucomyst® (acetylcysteine) for bronchodilation. If the Atrovent® Inhalation Solution (ipratropium bromide) is continued, I would give it at a different time than the Mucomyst® (acetylcysteine).
Hugs, Sunny