I have two sites, www.examplesite.com and www.secondary.examplesite.com.

I need some code that will make it so that when anyone
goes to www.secondary.examplesite.com/whatever it shows the same
content as if they had gone to www.examplesite.com/whatever. This is
not a redirect. The URL must remain www.secondary.examplesite.com
while users are surfing that site.

Each site is accessible within unix.

The first site:
/usr/www/users/example_user/main_directory

The second site:
/usr/www/users/example_user/secondary_directory