RewriteEngine On
RewriteBase /

RewriteCond %{HTTP_HOST} domain10.org$ [OR]
RewriteCond %{HTTP_HOST} domain11.org$
RewriteCond %{REQUEST_FILENAME} !dir1/
RewriteRule (.*) dir1/$1 [L]

RewriteCond %{HTTP_HOST} domain20.org$ [OR]
RewriteCond %{HTTP_HOST} domain21.org$
RewriteCond %{REQUEST_FILENAME} !dir2/
RewriteRule (.*) dir2/$1 [L]