Multiple installs, one DB, one install superior

I have a domain with one db and and two wordpress installations. The domain just had one wp installation before and I added the other one recently by adding the new tables with their different prefix. Each installation has its own separate folder.

The original blog works fine. The new blog lets me see its homepage when I visit the root of its directory (i.e. https://www.originaldomain.com/newinstall).

However, for all other pages (e.g. those produced dynamically by the install like https://www.originaldomain.com/newinstall/post) I get a 404 from the original install (with its original theme skin, etc.).

I can’t see what’s wrong.

The root index.php directs to the original installation’s folder:

/** Loads the WordPress Environment and Template */
require('./wordpress/wp-blog-header.php');
?>

The /newinstall/index.php directs to its own blog-header:

define('WP_USE_THEMES', true);

/** Loads the WordPress Environment and Template */
require('./wp-blog-header.php');

The settings all have the correct URLs to their own folders.

I wonder if there’s some sort of htaccess issue or something else I’m not seeing.

Is there anyway I can debug this, e.g. by looking at the exact handoffs taking place as the requests are made? I haven’t debugged WordPress before to that level.

Thanks!