The faq's on the website answer this https://www.bt.com/sport/box-office/faqs/
If you subscribe to BT Sport on a second Sky Q box, you’ll be able to see BT Sport Box Office on your Sky Q Mini. Otherwise, BT Sport Box Office will be available on your main Sky Q box only and will be unavailable on Sky Q Mini or Sky Q App devices, even if you receive other channels through these
