In vCenter 5.5 there are 3 deployment options, see this: Getting ready to upgrade production to vCenter Server 5.5? Make sure you're using the correct deployment option…
Basically you will need to select 3 on the 2nd, 3rd, and subsequent vCenter SSO installation for Linked Mode to work.
So the 2nd and subsequent vCenter join the same vCenter SSO domain (first vCenter).
See also this whitepaper: VMware® vCenter Server™ 5.5 Deployment Guide
Select Option 1 for the central site's vCenter and the remote site's vCenter select Option 3.
After that you perform the Modify Linked Mode configuration 7 vCenter + 7 vCenter.
There is a nice blog post that explain the steps in details here: Back To Basics: vCenter 5.5 with MultiSite SSO and Linked Mode Configuration « Mike Laverick…