It sounds like you want your instance of Discourse to be available on multiple domain names (same 2nd level domain but multiple first level domains (.net, .org, etc.)) and be able to log in to the forum from each one. So if one person is on your .net domain and another is on your .org domain, you want them both to be able to log into the forum? Is this correct?
I think that may require a plugin or implementation of a new feature to do the dame thing that’s presently done for SSO for Google, Yahoo, etc. that is already built into Discourse. This will have to be answered by a Discourse team member or a plugin author.