Yandex regards sites as mirrors if they are complete or partial copies of each other. Copies are detected by matching content on pages with the same addresses (example1.com/page.html and example2.com/page.html). For example, most of the time hosts like example.com and www.example.com are mirrors because the content on pages accessible using the same URL is identical.
Sites are also mirrors if there is a server redirect from the pages of one site to the corresponding pages on another site (example1.com redirects to example2.com, example1.com/page.html redirects to example2.com/page.html, etc.).
The robot groups mirrors together so that only one of them is indexed and participates in search — the main mirror.
If there is nothing pointing to which mirror should be the main one when sites are paired as mirrors, the robot chooses the main one automatically using its own algorithm.
Use the Host directive.
Use a server redirect from the pages of the old domain to the corresponding pages of the new domain (we recommend using this method if the new domain is not a secondary mirror).
You can see if a site is a secondary mirror of another site by adding it to this form: Report a New Site.
You can speed up registration of changes in Yandex.Webmaster.
You can check changes using the Report a new site form.
To check if a site is a mirror, use the Report a new site form. You will receive this message if a site is a mirror: “The site you indicated is a secondary mirror...”.
Mirrors can be separated as needed.
Yes, they are. This helps, for example, when migrating a site to a new domain. If the old domain becomes a secondary mirror of the new one, this will help the site maintain its search ranking.
Yes, the robot groups sites like this as mirrors, but we don't recommend that approach. It's better to just have redirects pointing to the corresponding pages on the other site.
That most likely happened because the new domain is already a secondary mirror. The old site lost its search ranking because the robot is getting a redirect code to a new site that has not yet become a main mirror and is therefore not listed in searches.
You can let the robot know that it needs to separate your sites using the Main mirror tool in Yandex.Webmaster.
You can check on the status of your request to separate sites using the Report a new site form. The separation is complete once you can successfully add a site that was previously considered a secondary mirror.