I have Battlefield Hardline for Xbox 360, and my brother has it for the Xbox One - can we play together online?
Unfortunately not, the differences between the hardware and servers of these two systems are just too great. You'll both need to have the same console in order to play with each other.
If you have other questions I've provided some links to help you out:
The creator of Battlefield wanted this initially, but decided the leave it afterwards, because of the community complaining about the advantage that the xbox one users will have above the xbox 360. Obviously the xbox one is quite alot faster than the 360. But the creator said he does not want to get stuck in a battle there and eventually bring down the actual performance of the game so everyone can play equaly