Transferring a site to another domain while maintaining both versions of the site, how?

Question

Google

Good day to all!
Asked the question of transferring the site from one domain to another. The sites will be absolutely identical, only the domain will change. I would definitely not want to use a redirect, because the task is such that both sites would work, but the new one has become the main mirror. In a few months, the old site will be closed.
Also, it is very important to transfer the site with minimal loss in search results.
I read the official information in Yandex and Google and would be very grateful if a person with experience could help sort this information out.
Questions:
1. Is it possible to achieve my goal without resorting to a redirect? If everything is clear with Yandex, then I didn’t find such information about Google, everything is tied to redirects, moreover, from each page to each.
2. Will the old site disappear from the search results after I write the host of the new site in the robotx.txt of the old site?
3. One of the conditions for gluing mirrors is that the content must be the same. How to deal with the information about the mail in this case. Example: on the site old.ru, the [email protected] was indicated in the contacts. After the transfer, the site domain, as well as the mail domain, will be changed to new.ru and the mail will also become [email protected] If I write a new mail in the contacts on the new site, will the search engine robots consider this a difference and will this not be reflected in the merging of mirrors?
I would be grateful for help in understanding these moments.

Answer 1

Just set up the new domain to work from the same place as the old one and that's it. Why transfer anything? A redirect will have to be done in order to transfer the reference mass to the new domain. And hardly a few months. Links to the old one will not move, users will click on links with the old domain in a year. Why not keep the second domain redirected to the new one until the last link dies ($10 a year question)?