Here is the official blurb.
"You can get BT TV in a second room with TV Everywhere with Extra Box, so you and your family can watch and record different shows at the same time. Plus you can watch TV on the go with the BT TV mobile app, which gives you access to a selection of entertainment channels, kids channels and BT Sport.
Here's what you need if you want to get TV Everywhere with Extra Box:
If you just buy a second box yourself, you will only be able to consume BT services on 1 at a time but can use the other as a Freeview HD recorder.
The maximum number of boxes currently allowed is 2.
If you have a third you will get full services but on only 2 boxes at any one time and may find some unusual behaviour.