What version of AsyncOS is running? As of 8.5 you should not need to add the feature key for centralized management. As long as you are seeing the clusterconfig command, you should be able to create a cluster, and then add your second appliance to the cluster --- as long as it is running the same revision of AsyncOS.
From the release notes for 8.5:
Feature key is no longer required to enable Centralized Management feature. By default, Centralized Management feature is enabled on your appliance.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...