A 3rd party option is the Ikelite Lite-Link. A Canon speedlite is placed on the Lite-Link unit. The Lite-Link detects both the start and end of any other TTL controlled speedlite, thereby adding the Lite-Link speedlite output to the light controlled by the camera. See the LiteLink web page: http://WWW.ikelite.com/web_pages/lite_link.html