Sorry there may be hundreds of posts about this.... however...
I have a now tv stick... 1 device I get that part....
Watching on x box =2
But why does watching on phone via chromecast count as 2 devices?
Do you have the NowTV app installed on your phone plus watching via the built-in browser? That would probably qualify as 2.
You can contact NowTV Support via online Chat to confirm.