No, this is not how SSO via MNet works. Every user in MNet network must have a single server where they log in (and are authenticated, for example against LDAP in your case). From this single entry point, they can roam to other servers. But they can't log-in at another host and roam back. They would be considered as different users and two user records would get created.
Example: imagine three MNet servers M1, M2, M3 (either Moodle or Mahara). M1 can be set up as a main entry portal where users log in against LDAP. Then all users are "local" users at M1 and they are "remote" users at M2 and M3. But if you enable LDAP authentication at M2 too, you get into trouble. The record for user who authenticates at M1 and comes as remote user to M2 is different from the record for the same user who authenticates locally against LDAP.