Multi instance Drupal on single server. Is SSO possible to be implemented?
I see that this answer is late but, yes.  I am setting that up now but still working through some config. 


I created a D7 installation called auth.local.domain.com (these are currently only on my dev machine).  For this instance I installed oauth2_server and openid_connect_sso modules
I created a separate D7 installation called client1.local.domain.com.  Here I installed the openid_connect and openid_connect_sso modules.
I installed 9 more sub-sites (client2 - client9) under Client 1, using drush.  
I used Drush to enable the openid_connect and openid_connect_sso for all sub-sites.
I added all 11 of the sites to my Apache config (I am using MAMP Pro on Mac) and tested them.
I had some config to do on on the auth site relating to each client site and to each sub-site for the SSO.  


I am using https://drupalcommerce.org/blog/32567/using-openid-connect-single-sign-drupal as my general guide for setup and configuration of the modules as well as https://www.youtube.com/watch?v=3K-2IfC8lpg but the multi-site part I figured out myself.