Re: Multi-Site SSO Setup vCenter 5.1 Linked Mode

What you will need here is to install SSO in site 2 in multisite mode.

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2034074

However, this treats each SSO as a separate instance.. if you want high availability(which is what you want in your case) there a HA mode as well

VMware KB: Configuring vCenter Single Sign On for High Availability

Or like Steven said.. wait for 5.5 to come out

Regards

Girish