Sept 14 (Reuters) - Canada’s main stock index opened flat on Tuesday, as gains in energy stocks were offset by weakness in healthcare, while data showing a drop in domestic manufacturing sales further dented sentiment. * At 9:35 a.m. ET (1335 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 4.39 points, or 0.02%, at 20,662.02. (Reporting by Amal S in Bengaluru; Editing by Maju Samuel)