I've read some previous threads about this subject (like this one, https://premium.wpmudev.org/forums/topic/multi-domains-single-sign-on-explained) but I'm affraid those haven't solved my problems.
As some other users have said before, what I'm trying to do is this:
I have two different WordPress sites on two different domains, for example to use easy names (I don't own those domains, of course):
Those domains are on the same server, but I wonder if they could even be on different servers.
Those sites allow user registration, and what I'd like is to enable single-sign-on, so the process for a user should be this:
1. He signs up on http://www.earth.com
2. That allows him to, for example, just comment on the site (not create new blogs)
3. He then opens a new tab on his browser and goes to http://www.mars.com, where there is an interesting discussion about water on mars he wants to comment in.
4. When he accesses this site, the site automatically logs him in because the single sign-on is enabled, so he can comment without registering or even login into the site.
Is that possible?
What I've done until now:
1. Created the two WP sites, each of them is a WordPress multisite, so I've got two networks, with just one site on each of them.
2. Installed the Multi-Domain plugin and configured it, so both domains (earth.com and mars.com) are public. And I've enabled Single Sign-on. The screenshot with the current configuration is here,
3. Installed the Domain Mapping plugin (I'm not sure if I really need this) and configured it with the server IP where both sites are. The screenshot with the config:
4. I've also installed the User Synchronization plugin, that works really well: when a user registers on earth.com, that data syncs to mars.com, so the user can login in mars.com even if he hasn't sign up on that site. I wonder if this is necessary, or should be something that the Multi-Domains plugin does with the enabled Single Sign-On.
I've created the (I think) mandatory A records on my domain register, so both *.earth.com and *.mars.com point to the same IP the two sites are on. I think this allows users to create new blogs on each network. In fact, I've registered a user called 'javipas' on earth.com, and he has been able to create the javipas.earth.com blog to publish what he wants.
But when the user javipas logins to http://www.earth.com, he expects to be automatically logged into mars.com when he accesses the site. This is not the case. I wonder what must I do to make this work.