Hi @PatrickSWarner, thanks for posting your question.
It is limited to just the one box mini. Have a look at: Can I watch BT TV in more than one room? | BT Help "We have limited Extra Box to two boxes to make sure you get the best possible experience"
Thanks for the confirmation. Interesting because for example if you have full fibre, I would have thought that there would be more than enough bandwidth for several boxes to operate simultaneously.