(Adds detail, background) TOKYO, Jan 28 (Reuters) - Japanese financial firms SBI Holdings and Sumitomo Mitsui Financial Group will launch a digital stock exchange to provide a trading platform alternative to the Tokyo Stock Exchange, sources said on Thursday. As a preparatory step, the two companies will set up a joint venture in Osaka in March, with SBI investing 60% and SMFG 40%, one of the sources said. The sources declined to be identified as the talks are still private. The Nikkei business daily, which reported the move first, said the new exchange will take orders outside the Tokyo Stock Exchange’s trading hours and will eventually handle digital securities. The Tokyo Stock Exchange currently dominates stock transactions in Japan. Trading on the bourse was brought to a complete standstill by a hardware failure all day in October last year, highlighting the need for an alternative platform. The new exchange aims to capture growing demand for transactions of digital securities, the source said.