First: Yes it is more advance than WPMUDEVS Q&A as Question2Answer is fully featured question and answering system offering more featues and have great flexibility to modify as per need.
For WordPress and Q2A SSO ( Single Sign On ) it will only work on the same server. The reason is when you install Q2A with WordPress as SSO Q2A will installed / create tables into the same WordPress database and will use WordPress user table ( default wp_usres ).
Second thing here you should understand the difference between Path and URL or Link. Path means local machine ( computer ) address ( here is your server ) and URL is same what you missunderstood with the domain name. http://www.domain.com or http://subdomain.domain.com is URL or Link which will never works for SSO setup.
I hope this will helps you to clear couple of confusion and missunderstanding about SSO, Path and Domain/URL/Link