You are talkiing about the 'Mirror' option in the Destination tab of a backup, is that correct?
I know of people who have mirrored to different tape devices, not sure if I have ever seen it used the way you describe. You should be able to do it, though, and, it makes some sense to do it this way
I was going to discount the licensing factor, but, now I am not so sure. DP Capacity-based licenses look at the total available amount of storage space, so I guess that this would include the Formatted capacity of the Tape Media . Usually, though, a Licensing problem will generate an error
Open a case with the response center (unless someone else out theer has other ideas) to have this problem looked at