WP is ignoring .htaccess rewritten URL

I have two domains being served from the same WP install. edit: not multi-site

In wp-config.php I have define('WP_SITEURL', "http://$hostname/wordpress"); etc.

The two domains will have similar content, but we want to be able to have them at the same path, e.g. domain1.com/about-us and domain2.com/about-us.

I’ve therefore set up these two pages dom1about-us and dom2about-us, and in my .htaccess I have:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /

RewriteCond %{HTTP_HOST} dom1 [NC]
RewriteRule ^((about-us|etc).*) /dom1/$1 [NC,QSA,L]

RewriteCond %{HTTP_HOST} dom2 [NC]
RewriteRule ^((about-us|etc).*) /dom2/$1 [NC,QSA,L]

RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

I get (edit: WordPress) 404 error pages for the above URLs. I’ve added print_r($_SERVER); in my 404 template and get the following (simplified output).

Array
(
[REQUEST_URI] => /about-us/
[REDIRECT_REDIRECT_STATUS] => 200
[REDIRECT_STATUS] => 200
[HTTP_HOST] => dom1.dev
[SERVER_NAME] => dom1.dev
[REDIRECT_URL] => /dom1/about-us/
[QUERY_STRING] => 
[SCRIPT_NAME] => /index.php
[PHP_SELF] => /index.php
...
)

Doesn’t that mean that the rewrite has worked, but WP is not using it?

Many thanks in advance.