Do both the buyer and co-buyer have to be on same insurance policy even if one never drives the car?

As the cosigner has a financial interest in the property, meaning if something happens to the vehicle he can be made to pay "All" the bills, Then he should certainly be insured on the policy for any loss regardless of whether he is driving it.

A cosigner is jointly and severally liable to the lien-holder for the full amount of the note and is probably required to be insured under the finance contract anyway. The cosigner if unlicensed and does not drive should at the very least be listed as a holder of interest in the lien-holder section so that they will receive notification from the insurer should the primary have any lapse in coverage.

The primary driver should also pay any additional premium required to insure the cosigners interest. After all, he was kind enough to co-sign so you could buy the car, the least you can do is make sure he doesn't wind up in the poor house should something happen to it.

Should the primary Driver / borrower refuse to insure the cosigners interest, it would probably be prudent for the co-signer to take legal recourse against the primary borrower before a loss occurs.