Hi @Ms67
Before July 2021 you could watch two simultaneous streams at the same time (this has now changed to one stream without the additional Boost Membership).
Should you be a BT TV customer then you can still watch two devices at the same time (without Boost) by following a certain sequence of watching on your devices.
Have a read of this link article below about managing your devices, which should explain things in more detail.
https://help.nowtv.com/article/managing-my-devices