I always recommend you discuss with your account manager at Tableau versus taking a response on the forum (from someone like myself who isn't affiliated with Tableau directly) as gospel
pinki sharma You can use the same license key of your primary to the backup. Usually Tableau server license will be used for one prod and 2 non-prod environments. Here Primary & Backup machines ..either one system will be active and the other will be in Passive mode ..so you are good to use the same primary license key for the backup.
Yes Tom W agreed with you, it all depends on what deal your company got from Tableau sales team when they bought the server.
As Tom states - have a discussion with your account rep regarding licensing issues. They are here to help.