A tentative contract agreement was reached for the second time in a week in the strike by 2,700 San Francisco janitors. Spokeswoman Jean Qwan of the Service Employees International Union Local 87 said the agreement was about the same as that reached over the weekend with the help of Mayor Dianne Feinstein. The earlier settlement fell through when the union complained that the San Francisco Contractors Assn. reneged on several key points. There was no comment from the contractors. Qwan said the janitors will not vote on the new contract until they are certain that management has voted in favor of it.