The concert will be at Terme di Caracalla on Sunday, July 31, 2016
Tickets went on sale at 10am Central European Time on Wednesday, June 8 on Ticketone.it (online and retail outlets), and at the box office of the Teatro dell'Opera di Roma. Online links to the box office redirect to Ticketone.it.
This link works. The one on the 2CELLOS website is currently going to an operatic performance unrelated to 2CELLOS.