Cross domain single sign on has been a nightmare for us—we’ve had dozens of highly experienced Wordpress programmers try to do it for us with no success.
Your own support forum highlights the difficulty:
We would like to know if your domain mapping plugin has recently solved this problem, or if you mean something different by “cross domain cookie syncing.”
We have 500+ domains (not subdomains).... domain1.com, domain2.com, etc. All using a single multisite installation. Our requirement is to allow users to go from one domain to another, and only have to log in once in the beginning. We've been successful with SSO only with subdomains of a single domain, but not multiple domains.
We explored this with you 2 years ago rather intensely, and ultimately concluded there was a serious scaling problem, and had to use the Wordpress domain mapping plugin instead.