I don't believe this is possible. Security contexts can be thought of as completely separate firewalls. So, just as two physical ASAs cannot share configs (excluding failover), two contexts have no way to share configurations.
As you mentioned, you'll have to manually change configs for both contexts.
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...