If I co-own a vehicle with my 35 year old son and am secondary on the auto policy, will I be equally liable in the event he has an accident?

There is a difference between YOU being liable, and your insurance company being responsible under your policy.  The person who would be liable would be the person or persons whose negligence caused the accident, usually the driver.  If the driver was not the owner but, for example, had a bad driving record, it is theoretically possible that the owner could also be partly responsible for allowing a dangerous driver to use their car.  Assuming that your son is driving and that there is no reason why he should be considered an especial risk, he would be liable for damages incurred in an accident caused by his negligence but you, not having caused the accident through negligence, would not be.  His insurance (on which you are also an insured) would cover such damages subject to the terms of the policy, i.e. there may be deductibles, policy limits, and particular types of damages which are not covered, and for which your son would be fully responsible.  In addition, your son's insurance premiums would go up.  I'm not 100% sure, but I believe that your insurance premiums wou also rise in such an instance.  Whether your separate insurance would also be used to cover your son's liability depends on the language of that contract.  For example an umbrella or excess policy might extend to such a claim, but I do not believe that an ordinary automobile policy on Car A would cover an accident in which Car B was involved, even if Car A and Car B are owned by the same person.  Again, however, exactly what your policy covers depends on the specific provisions of your policy.