You can read this blog on vCenter 5.5 SSO: Allow me to introduce you to vCenter Single Sign-On 5.5 | VMware vSphere Blog - VMware Blogs
"The new architecture is based on a multi-master model where each instance is automatically kept up to date with it peers via builtin replication. "
Also read this blog to understand the vCenter 5.5 SSO deployment options before installing: Getting ready to upgrade production to vCenter Server 5.5? Make sure you're using the correct deployment option…
vSphere 5.5 Documentation Center - vCenter Single Sign-On Deployment Modes
There are 3 deployment mode and deployment option 3 is required for Linked Mode.
"Option 3 - Multiple Single Sign-On instances in different locations: This deployment mode is required if you have geographically dispersed vCenter Servers in Linked Mode."
"Deploying multiple vCenter Servers either local to each other or geographically separated, say for use with Linked Mode ... by selecting deployment option #3 vCenter Single Sign-On for an additional vCenter Server instance with a new site. This selection will create a replication partner and have no dependency on the first vCenter Single Sign-On server deployed."
Theres also a whitepaper on vCenter 5.5 Deployment Guide: http://www.vmware.com/files/pdf/vcenter/VMware-vCenter-Server-5.5-Technical-Whitepaper.pdf
Read also these links:
vSphere 5.5 - SSO Multi-Site Deployment - Learning by Doing | vTricks.com
vSphere 5.5 Documentation Center - Linked Mode Considerations for vCenter Server
If you need to uninstall and reinstall vCenter Server on more than one member of a Linked Mode group, do so with a single vCenter Server at a time. Uninstalling and reinstalling multiple linked vCenter Servers at the same time is not supported, and can cause errors that prevent vCenter Server from connecting to vCenter Inventory Service. If it is necessary to uninstall and reinstall multiple linked vCenter Servers at the same time, isolate them from the Linked Mode group first, and rejoin them to the Linked Mode group after the reinstallation is complete.
VMware KB: Upgrading to vCenter Server 5.5 best practices
"Before you upgrade any vCenter Server that belongs to a Linked Mode group, remove it from the Linked Mode group. Upgrading vCenter Servers that are members of a Linked Mode group can cause the upgrade to fail, and can leave vCenter Servers in an unusable state. After you upgrade all members of a Linked Mode group to version 5.5, you can rejoin them."