Passwords alone won't be enough to get onto America Online under a new, optional log-on service that makes AOL the first major US online business to offer customers a second layer of security. The so-called two-factor authentication scheme, being unveiled on Tuesday, will cost $1.95 a month in addition to a one-time $9.95 fee. It is initially targeted at small businesses, victims of identity theft and individuals who pay a lot of bills and conduct other financial transactions through their AOL accounts.