we want to install an SSL certificate on our site, but we are not planning to switch from HTTP to HTTPS yet.
The question is: Will a mirror site of HTTPS appear in search engines after installing the certificate? Will there be any troubles in issuance or with traffic?
If the site with HTTPS will be available only with direct access. All links, as well as robots, sitemap, data in Google Webmaster remain unchanged.
If your site is hosted somewhere, then most likely it will be possible to access the site immediately and via https.
If you host yourself, just don't write the https section in nginx or apache.
And which is the problem with installing a certificate just when you need it?
Technically, two different web sites can be hosted on the same domain over HTTP and HTTPS. Their root pages may match "completely by chance".
The Same Origin Policy considers different protocols to be a sufficient difference to prohibit access to resources without explicit permission.
If the web sites are completely different, it is more likely to confuse and make it difficult to use things that involve "raising" links to HTTPS (say, HSTS).
These protocols use different ports by default, 80 and 443 respectively. Due to DNS features, both protocols must be served by the same machines, but within the machine, ports 80 and 443 can listen to two different processes configured differently. This is in theory. In practice, I have not met that.
In practice: in the nginx configuration file, you can make one block with listen 80, specifying there what to serve over HTTP, and in the block with listen 443 ssl, specify the settings for HTTPS.