I've got an older BT router (about 2 years old) and 3 white discs, all connected to the router by ethernet. All work fine although there are a couple of black spots in the house. I've been persuaded to get a Smart Hub 2 and a black disc.
I understand that at least one white disc has to connect to the Smart Hub 2 via ethernet. But can the white and black discs both work as part of the same network?
No, the two systems cannot work together, you either have to use one system or the other.
As you already have the white whole home discs, there is no point in using the black Complete Wifi system unless you intend to stop using your existing system and move over to the BT Complete Wifi Guarantee.
I doubt very much you will get the same coverage using the SH2, and just one black disc.
You could add an extra white disc to your existing system and allow it to link to the nearest disc.
Unless you must have a mesh system, wireless access points are a much cheaper option if you have existing Ethernet cabling.
Oh, that's really annoying as I specifically asked that question (could the white and black discs mix) when I was sold the the Smart Hub 2 router and disc by the BT person.
They wont work on the same network without complications unfortunately.
I would replace your setup and switch to Complete WiFi package with your Smart Hub 2 and Wifi Disc and they'll send you out more discs if you need them.
Hope this helps,