i fixed it using the apache proxy module:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{QUERY_STRING} !location
RewriteRule ^(boston|newyork)/$ /index.php?location=$1 [NC,QSA,P]
RewriteCond %{QUERY_STRING} !location
RewriteRule ^(boston|newyork)/(.*)$ /$2?location=$1 [NC,QSA,P]
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
the [P]
option does the trick.