MNet is designed so that user has to always log in at their home server first and then they can roam to remote servers. In your case, users has to log in at Moodle A and then click a link to their remote courses at Moodle B. At server B, they are logged in automatically (that is what SSO via MNet does). But note that they has to use a special link to that B server (generated by Peer servers block or My courses block, for example) so that SSO actually happens.
d.
Moodle networking (MNet)
Login in on a remote site
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