Do allergies usually affect one or both eyes?

Both: Allergic conjunctivitis is typically the result of airborne allergen impacting the eye and causing symptoms. B/c the allergen is airborne it will "hit" both eyes. It is distinctly unusual for one eye to be effected.

Both eyes: It is not easy to explain eye symptoms triggered by airborne substnaces affecting only one eye. Unless the exposure was brief and there was protection over the unafffected eye.