Does a motorcycle have to have both mirrors in California?

No it only ha to have 1 mirror.

26709. (a) Every motorcycle subject to registration in this state shall be equipped with a mirror so located as to reflect to the driver a view of the highway for a distance of at least 200 feet to the rear of such vehicle. Every motor vehicle, except a motorcycle, shall be equipped with not less than two such mirrors, including one affixed to the left-hand side.