If two customers hand over IDs but one person's ID clearly belongs to someone else, the server should:

If an ID clearly belongs to someone else — the photo or physical description does not match the person presenting it — the server should decline to serve alcohol to that individual, because they have not validly verified that the person is of legal age. Using someone else's ID is a common tactic by minors. The server need not make accusations but should simply not complete the sale without valid identification. Following this rule protects the server from the serious consequences of serving a minor. When in doubt, do not serve.