“Mr. Nelsons, 36, will continue to lead the Boston Symphony, which recently extended his contract through at least 2022, after he begins his five-year contract at the Gewandhaus Orchestra in the fall of 2017. … Boston and Leipzig are working to make a virtue of Mr. Nelsons’s involvement with both orchestras. The Boston Symphony will perform in Leipzig, and the Gewandhaus Orchestra will perform at Symphony Hall in Boston. They are starting a joint commissioning program that will begin in the 2017-18 season.”