Hi,
As you have mentioned " I have an agency that has an active/active cluster which uses BackupExec and wants to become a SAN media server in NetBackup. " whereas "NetBackup can only be installed as an active/passive cluster" ..
It is not recommend installing any other Backup app on the same server as a NetBackup master/media server
Also if you have the DataTapes of Backup Exec , you still can restore using NetBackup as NetBackup can accomplish restores of BE tapes,by importing (catalog) BE tapes in NetBackup.
But if you want both the applications BE and NBU running the same server.... it is not possible ..
Hope this will help you
Thank you
Gauri