Some insurance companies will cancel your policy for misreprentation if you do not disclose your teen driver during the application process or when he or she gets a driver's license. Even worse, if you file a claim for an accident your teen has, your premiums could go up as a result. And then your insurer could nonrenew you. Some insurers require that all drivers be named on both policies, no matter how much time they spend in a household. Whether a child is at the house on the weekends or full-time, if the visiting teen customarily operates the covered vehicles, the insurer has the risk exposure and thus they charge for that exposure.
Parents living in separate states introduce another layer of complication.
Should the parent without custody allow their teen to drive their vehicles without disclosing it to their insurance company, they would likely pay a surcharge should the company find out. Then you're stuck paying out of pocket property damage and bodily injuries from the accident. If you're unsure whether your child is adequately covered if he or she uses either your vehicle or your ex-spouse's vehicle, contact your insurance agent and ask. Otherwise, you may be hit with surcharges if you file a claim in which your child is found at fault.