We have BT broadband (fibre to the premises giving 76mbps) and BT TV with the BT Sport and Sky Sport addons. I know that we cannot have an "Extra box" as we have Sky Sport, but am looking to replace an old and unreliable Humax PVR in the bedroom.
The BT Youview Plus recorder box is an obvious possibility, given its size, silence and cost, but I have read a review that says that a second BT Youview plus box will not work where you already have one such box connected to BT TV. I can understand this as regards the BT TV channels, as the subscription only covers the original box, but won't the second box just work like a PVR as regards the normal (ie non BT TV) Freeview channels?
It will work just fine. Just no BT services. Though you will be able to switch BT TV from one box to another. Just not both at the same time.
I've got 3 on the go at the moment with no problems.
Personally I'd hold off getting another Youview box right now as there are numerous reports of failed recordings & I do mean numerous.