I did exactly this with a new NowTV Stick a few months ago.
Ensure, though, that you sign in on the Stick with your existing account details.
The two-month Entertainment Pass will then go and live with, but be separate from, your existing Pass; and if/when that has less than a month to run, you can Redeem the new Pass, which will cover the next two months after your existing Pass expires, after which you will go back onto the normal payment cycle.
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