Mixture
No need to get a 2nd BT Broadband line. As long as the existing BT broadband account is "Opted in" to BT WiFi then the BT HomeHub will broadcast a BT WiFi with Fon and a BT WiFi SSID along with the normal hub SSID.
Anyone who sets up a BT WiFi account can then log into the BT WiFi service and get speeds of between 0.5 and 3.0 Mb/s. None of this usage counts against the owner of the broadband account. Up to 5 IP addresses are available on the BT WiFi from a HomeHub. The broadband account owner always gets preference over the available bandwidth.