since the passive is unique on the arpen can or can't i have a black cleaver and then a brutalizer equipped?
The passive armor pen doesn't share a passive name and they are different items.
Therefore they both will work.
Many AD casters will build a BC, then a brutalizer still and then move on to other itemization.
© 2013 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.