Legal Question in Personal Injury in California

It's not so much a matter as who is on the policy, but in a typical auto collision case, the plaintiff sues all of the owners of the other vehicle.

An auto insurance policy doesn't make someone liable, it protects them from liability. The question is why are both people on the policy. If they are both on the policy because they are married or domestic partners, and one of them caused an accident, to the extent the insurance doesn't cover the loss only the person who caused the accident is actually liable, but the other spouse/partner's share of their community property can be taken along with the liable party's share to satisfy a judgment. Also, if they both own the car together, in California if you own the car you are jointly liable with anyone legally allowed or entitled to drive it if they cause an accident. Parents may be liable for accidents caused by their children, etc. So bottom line: the insurance policy has no effect on liability. It is the connection between the two people and each person's connection to ownership of the car that affects who may or may not be liable.