Share login status between multiple hostnames

The story:

My forum’s domain got DNS-poisoned by my university ISP, so I registered a new domain and use it for Discourse. The old one is still in use(had been renewed for several years to expire) by utilizing @michaeld 's discourse-multiple-hostnames plugin and reverse proxy server.

The problem:

How can I share the login status between multiple hostnames (though they are of one instance)? I know a simple solution to my problem is doing 301 redirect, by I do find this status sharing useful, e.g. using multiple reverse proxy servers in different hostnames with different BGP routing to the source server.

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