As we age, we may become intolerant to anticholinergic drugs.

We become more sensitive to their effects. Both drugs have this side effect.
With dry eyes/mouth issues, this tends to come with age too.
So when you add them together... it becomes a new "side effect".
Nortriptyline is the active metabolite of amitriptyline...so I would expect some of the same effects. I'd discuss with the doctor, and see if a dose reduction may work still. Also this side effect would be additive with Benadryl and other OTC first generation antihistamines, like Chlortimeton and Unisom. Other OTC drugs like Imodium may be additive too.

It is also possible you are developing Sjogren's syndrome which is an autoimmune disease affecting moisture producing cells in the eye and mouth.