Treasury Secretary Lloyd Bentsen denied he knew anything about contacts between his top staff and the White House concerning the Whitewater investigation until after those meetings occurred. Bentsen testified before a Senate committee holding hearings on the Whitewater affair. As Bentsen took center stage in the latest day of the proceedings, his deputy, Roger Altman, was being grilled again on the same subject, this time by a House panel. Altman has been through the wash -- the long cycle. He spent 10 hours in front of the Senate committee, winding up at 2 o'clock this morning. Altman insisted again today that neither President Clinton nor officials close to him had done anything to derail the investigation. He emphasized he would not resign.