Complex sentence: In 1957, the Stars merged with the Takahashi Unions to form the Daiei Unions, who a year later merged with the Mainichi Orions to form the Daimai Orions.
Split into simpler sentences: In 1957, the Stars merged with the Takahashi Unions to form the Daiei Unions. In 1958, the Unions merged with the Mainichi Orions to form the Daimai Orions.