Sub section of the relevant FAQ copied & pasted below.
If I buy the fight to watch via my PC/laptop/MAC, could I then chose to watch it via my NOW TV Box/Smart Stick or through the iOS or Android app?
Yes, absolutely. You can use the same login details to access the fight through the Sky Sports Box Office app on NOW TV or through the iOS and Android app. Please note that you can only view via one platform (i.e. website or NOW TV or iOS or Android app) at a time - you won't be able to watch in parallel.