Before enrolling in a second medical or dental plan, you need to verify with each plan how the "coordination of benefits" will work--determine which plan is primary and which one is secondary. You need to check this coordination not only for yourself, but for each dependent that is to have dual insurance coverage.
If you are currently covered by your spouse's or domestic partner's non-CalPERS sponsored health plan and you are considering enrolling you and your eligible family members in a CalPERS-sponsored health plan, then you need to know how this affects your coverage and that of your dependents. Will the new enrollment change the physician, specialist, and/or hospital that you or your dependents can go to for services? Are there certain services for which benefits will not be coordinated? Find out in advance of enrolling in a secondary plan, so you may choose the right plan that works for your particular needs.