Updating Account "Bill To Account #" when entering a "Bill To Account # within Contacts"

Updating Account "Bill To Account #" when entering a "Bill To Account # within Contacts"

I have "Bill To Account #" on my Account page as well as my Contact Page.  Is there a way to update both when I enter in one or the other?  Automatically update the other page so I don't have to enter it twice?
thanks,