First you need to install vCenter_1: SSO>>Web client>>Inventory service>>vCenter server componets.
Once vCenter_1 installation is done. Start vCenter_2 installation>> do not install SSO again>>start web client installation>> whenever there is window to add SSO details>>Point it to SSO from vCenter_1. Register all other components to SSO from vCenter_1
You can re-point one of your existing vcenter server to other sso to make it consistent. Here is a KB article on how to achieve this VMware KB: Re-pointing and re-registering VMware vCenter Server 5.1 / 5.5 and components
Are you trying to configure the 2 vCenter Servers in Linked Mode? If so, keep in mind that you need to use the SSO multisite deployment mode. http://pubs.vmware.com/vsphere-55/index.jsp?topic=%2Fcom.vmware.vsphere.install.doc%2FGUID-C6F362AF-...
Mike Laverick wrote a good blog post about this Back To Basics: vCenter 5.5 with MultiSite SSO and Linked Mode Configuration « Mike Laveri...
I check vCenter Server in the Control Panel. I saw vCenter Single Sign-on(installed), VMware vCenter Inventory Service(Installed) VMware vSphere Web Client(Installed). But why in my vCenter Webpage, I did not see SSO.