Have you upgraded from vCenter 5.0 to vCenter 5.1?
If yes, you can add several domains as Identity Sources in SSO: http://pubs.vmware.com/vsphere-51/index.jsp?topic=%2Fcom.vmware.vsphere.security.doc%2FGUID-B23B1360-8838-4FF2-B074-71643C4CB040.html
Have you upgraded from vCenter 5.0 to vCenter 5.1?
If yes, you can add several domains as Identity Sources in SSO: http://pubs.vmware.com/vsphere-51/index.jsp?topic=%2Fcom.vmware.vsphere.security.doc%2FGUID-B23B1360-8838-4FF2-B074-71643C4CB040.html