Files sites are in directories s1 and s2.
When contacting site.ru
This is a very bad mnogosaitov.org - made crooked.
Need to do at least this:
/home/bitrix/www/.top.menu.php
/home/bitrix/www/about/index.php
/home/bitrix/www/s2/.top.menu.php
/home/bitrix/www/s2/about/index.php
And then the first site open for site.ru (SITRE_DIR = /)
And old.site.ru make SITE_DIR = /s2/
Or do different folders post:
/home/bitrix/www/* is the first website
/home/bitrix/ext_www/s2/* the second website
What You have done, maybe to dev'and fit, but not for production - you SEOs sticks in the dark score