Teller Cash Vault Combinations

Does anyone have any input into how many numbers should be required for a teller cash vault combination? I have been in banking for over seventeen years and have always seen a minimum of three numbers used for vaults (ex. teller cash vaults, manager's vaults, etc. all inside the main vault). For some odd reason my Security Officer wants to set up one of our new branches teller cash vaults with only two numbers (he originally was going to do only one number when I adamantly objected). I am trying to convince him to stick with at least three however I am at a loss for a really good concrete argument as to why three is best. Any ideas would be greatly appreciated.