You would never have a scenario where, as you put it, the Admin context would monitor Gi0 and ContextB also monitor Gi0. This is because you need to assign the interface to a specific context and once it is assigned to one context it can not also be assigned to another...unless you have configured subinterfaces, then those subinterfaces can be split up and assigned to seperate contexts. But one interface or one subinterface can not be assigned to more than one context.
Now, if you have failover groups configured and an interface on one failover group dies, then only the context that the interface belongs to will failover to the standby failover group.
The following is a good article to have a read through on the Active/Active failover functions:
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...
[toc:faq]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 and UDP are I...