The information should be listed in the log somewhere, if you look through it, though it's not as clearly labelled as the initial block of information where you're told which tape, media set etc is being used. As far as I remember it's almost hidden amoungst the other information, more as an aside than a notification, kind of a "oh by the way, I've changed tape".
If you can't find it you could try looking in the restore selections at the different drives etc being backed up by your job, since each of these will display which tape was used by them. While I think they only display the tape that was used to start that backup, with the amount of data you have I would imagine at least one of the drives / system state / exchange bits would have started on the second tape, so it will show up.
