Unless I am missing something (very easy for me) why would you need to subscribe to a second box subscription if you can view content on a Now TV Stick or App as well as the BT TV box?
You don't.
You only need second box subscription if you want two boxes the dubious benefit of which is that all channels are accessible from the EPG and (depending on which box you go for) the capability to record.
If you're quite happy to watch the channels through a Now TV stick or Roku box or whatever then the second box subscription is unnecessary.