Hi ,
As per my knowledge the netbackup will not give any pop-up message for extending the usage of media. In order to make netbackup to use the available media for the backup, you firrst need to create a volume pool (Media and Device management -->Volume Pools-->rightclick-->new). then you have to move those 2 tapes to the volume pool which you have created. (By just right clicking on the media, select change and click on new pool...) You will be getting the volume pool which you have created, just select it and give OK. Now you have mention this volume pool in the schedule which is there in the policy.....
After all the above configurations, the netbackup would have the capability of using the second media if the first is completed.
Please reply if you still not getting the point.
Thanks
Dinesh.