I've got two WordPress MU sites: two different networks, two different domains:
If a user registers on earth.com, is there some way to make this user logged also in mars.com, for example to comment on post there or download a PDF available only to registered users?
The idea is to enable Single Sign-On (SSO) on these sites so once a user has registered and login into one of the sites, he will be logged on the other(s).
If this is not possible, I think it would work with subdomains on a unique WordPress MU site (network), right? How does this work, what must I do in this case?