Insurers don't determine your actual cash value (ACV) settlement based on what you owe, but rather on what the car is worth just prior to the accident. Let's say you owe $20,000 on your new car, but it's only worth about $16,000. If your car is totaled, you might get a settlement check of $16,000 but still owe an additional $4,000 on your loan or lease.
For serious accidents, you want enough insurance to cover a judgment against you in a lawsuit without jeopardizing your personal assets. Therefore, it's a good idea to have the same level of bodily injury coverage for all your cars. You may also want to consider an umbrella policy which provides additional coverage for more serious accidents and lawsuits.