Secondary signs. If an establishment has a direct entrance into the establishment in a wall other than the front wall, there may be a secondary sign affixed to such wall, and if an establishment has a wall, other than the front wall, that faces upon a street or parking area, there may be a secondary sign affixed to such a wall; provided, however, that no establishment shall have more than two secondary signs in any event. This display area of all secondary signs shall not exceed one square foot for each linear foot of the walls or 40 square feet, whichever is less.