Hi,
This is by design and no way to change it.
See the following TechNet page:
How DPM Uses Tape Libraries
http://technet.microsoft.com/en-us/library/hh757885.aspx
Short-term tape "full and incremental" option
The full backup will require a free tape for each scheduled job, and the incremental backup for all data sources will be appended to a single separate tape.
As tapes fill up, new free tapes will be allocated.
When the next full backup occurs, it will require another free tape, and subsequent incremental backups will be appended to another free tape.
Please remember to click “Mark as Answer” on the post that helps you, and to click “Unmark as Answer” if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread. Regards, Mike J. [MSFT]
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