Will both irises dilate and shrink in unison if one eye has an opaque cataract but not the other?

Normal yes. It is important that both irises respond in unison as described. If there is a disconnect, it can be a sign of more serious conditions and should be evaluated by an ophthalmologist/neurologist.
Yes. The control of the pupil arises from the brain and is simultaneous, equal and bilateral. The presence of a cataract in one eye does not change this.