Hi Dsohayda,
At the moment, I can confirm that if you want to join 2 vCenter instances :
1. You will install first vCenter: SSO>>Web client >>IS >>VC
2. While installing another vCenter:: You will register second VC with first SSO. Other components would be separately installed on second.
Hence there is only one SSO.
I am digging into all other queries. Will be back.