That's a good question.
Your best option would be to get the official answer from NowTV.
If you go to My Account > My Package there is a Telephone helpline number for Broadband enquiries.
Let us know what they say.
You must return your NOW TV smartbox if you cancel your NOW TV Combo in accordance with this contract.
NOW TV smartbox is limited to one per address per 12 month period.
I found this in the contract for Combo and is under section kit 4.1.