MHi @Anonymous User
I am not a BT TV customer, but there is some information about how to watch on two with non Boost or three streams (if you have Boost) from this BT TV FAQ help article linked below.
To watch on two devices at once:
If you're watching on a BT TV Box, you'll need to leave NOW channels to start watching on the App. Then you can go back to watching NOW on your BT TV Box too. You can watch live on your BT TV Box, plus live or On Demand via NOW App.
If you're watching on two BT TV Boxes then you can start them in any order.
If you have NOW Boost or the HD add-on, you watch on up to 3 devices at the same time