Can I use a 24 volt battery bank on a 12 volt starter motor?

When a 6 volt car is converted to 12 volt they often leave the 6 volt starter. It works good, real good. The starter's life is shortened and they install a 12 volt unit when/if the 6v starter fails. No big deal. Used to be real common conversion many years ago. Worst case problem would be if it had trouble firing up, that's when the starter is in trouble. I'd run it.