I guess first I need to tell you what stacking is. Stacking is the proces whereby you take two insurance policies and add the limits together. So, if I am driving your car and my limits are $100,000 per person and your limits are $300,000 per person, if you stack the limits, an injured person could receives $400,000. Make sense?
The answer in California is generally no. You cannot stack policies. Some states allow it, and in certain odd situations, you may have something similar to stacking occurring, but for the most part, you cannot stack policies.