Hi Fabrizio, welcome to the forums.
The fact that multiple media sets are created I think should help you confirm that it's using Parallel Processing.
As noted in the User Guide, parallel processing will create and track a media set for each drive in use during the job, so you can confirm from the HTML report on the tape job if there are multiple media sets being used during the job run.
See the Important Tip on that page:
"When multiple media sets open simultaneously, they may have identical sequence number and time of creation. To distinguish between the media sets easily, use the %id% variable in the media set name. This variable is added to the media set name by default."
I think that's easiest way to track it, and it will be visible in the HTML report as well, focusing on the Media Set IDs.