California bans stacking by statute
California's rule is in Insurance Code 11580.2. Subdivision (q) bars adding together the limits of two or more vehicles or policies, subdivision (p) sets the underinsured motorist rules, and the minimum limits are in Vehicle Code 16056.
Read why California does not stack coverage, check what subdivisions (d) and (q) say, or see how the limits-to-limits underinsured test works.