Primary Domain Examle.com is on CMS Drupal 8.
I must to share host with other site on different CMS location
What must I change in
.htacces to have access to othersite.com, and not break anything?
Create a new folder for the new site with your webhosts FTP solution, for example named "drupal-8-site".
Place all the new files in this folder and add the following line to the .htaccess file:
Try to clear the cache and see if the new Drupal 8 CMS works as expected.
With this solution you don't risk breaking the other site, as you aren't really changing anything in the othersite.com folder.