According to the document here (pg 11), "If you want to recover more than one database at a time, you can add multiple databases to the recovery storage group as long as they are all from the same original storage group (once you have added the first database, you can only add databases from that database's storage group). Otherwise, you must use more than one recovery storage group (on more than one server)."
--Full disclosure, I haven't done restores with Netbackup. The Exchange restores that I've performed were with a different product and procedures. Regardless, I think that you'd still be restricted to the above.