domain mapping and multi-domains single sign on

Hello!

I am having some trouble with single sign on for my users in multisite. I have both domain mapping and multi-domains plugins installed and i would like a user to be able to sign in at x.com (or subdomain.x.com) and be automatically signed in on y.com (or subdomain.y.com). ie completely sync login cookies across all domains and subdomains in the multisite network.

I have enabled 'single sign on' in multi-site settings. I have also enabled 'cross domain auto-login' in domain mapping.

Although i am able to login to x.com and automatically get logged into subdomain.x.com, it does not automatically log into y.com or subdomain.y.com. I hope that makes sense? Ie cookies are syncing to subdomains but not across top level domains. Is there something i am missing in trying to achieve this?

PS : I have tried with and without activating asynchronous autologin in multidomain plugin, it doesnt seem to change anything. I have also used a plugin called 'multisite user management' to ensure that all users do have at least a subscriber role on all sites.

Any help would be very greatly appreciated. I have opened support access to the site if it would be easier to take a look?

Thanks very much in advance

Matt