I tried assigning the url key with an array and surprisingly the resources loaded successfully on both domains.
'url' => ['http://forum.zsto.xyz', 'http://zsto.chtss.cn']
However, the server returned 404 error upon user registration. Other features seem to work perfectly.

    ecardm Flarum is not designed to work with multiple domains. You should decide which URL will be your canonical URL and setup HTTP redirects to that one. You can setup that using webserver rules or with an extension like Canonical Url.

        clarkwinkelmann If I setup canonical url, will the user be redirected from the other domain to the main domain or will it just redirect the AJAX HTTP requests to the main domain and stay on the other domain?

          You can do the something like this in the config.php; please note this might have unwanted side-effects.

          "url" => "https://" . \Illuminate\Support\Arr::get($_SERVER, 'HTTP_HOST', 'fallback.com')

          This will set the URL to the currently requested hostname and use fallback.com in cli or when none has been identified.

            luceos Sorry I tried 'url' => 'http://' . \Illuminate\Support\Arr::get($_SERVER, 'HTTP_HOST', 'forum.zsto.xyz') and now the site is not responding even if I change back to 'url' => 'http://forum.zsto.xyz'

                It's important to note that with luceos suggestion some extensions might stop working or require additional setup, in particular social login extensions.

                It might also cause worse SEO due to duplicated content.

                    luceos Thank you for this solution! Now it's working.

                      clarkwinkelmann Thank you the notes. I only need the other domain for internal use (WeChat app blocks non government approved sites) so it's fine.