I'd recommend connecting the Youview+ box first. Go to the extra channels and make sure you can get the channels. Afterwards, reconnect your other box.
For our BT TV Extra Box service:
BT TV Extra Box
If you’ve got a recordable YouView box, you can order an extra one to get BT TV in another room. Prices start from £5 a month, plus £49 for a YouView box or £99 for a YouView+ box.