You still should be able to.
I recently bought a second Youview box and all of my purchases from the BT Player were on this box aswell.
How do you have the second box connected to the internet? Is it connected to the same network as the other Youview box?
BT TV terms and conditions state it will only work on one box which i think is pretty poor:
If we have provided you with an extra set-top box to watch the service in another room, when you buy buy-to-keep content on one set-top box, you won't be able to watch it on the other.