Paul,
  The fact that you get half of the speed of a single backup for a ITC
backup may be telling.  Since ITC does it's duplication on the client
(i.e.: two backup streams to the media server) as opposed to on the
server (i.e.: one stream from the client and two streams to the media
from the server), you may be running into a communications link
limitation going from the client to the media server.

Mark