Each lot shall have two side yards with a total width of not less than 16 feet, neither one of which may be less than six feet in width at the front and rear building lines. When a lot less than 60 feet in width exists in separate ownership on the effective date of this chapter, the total width of the two side yards may be reduced by six inches for each foot of lot width less than 60 feet, but not to a total width of less than 10 feet. When the total side yard width becomes less than 12 feet, the two side yards shall be of equal width.