Install it at the root and use a custom theme for the first site.
Specifically, you want a subdirectory install, and then for the root sites theme, replace the contents of index.php with that of your static html page. You’ll need a style.css with a comment at the top so WP picks it up as a theme, there are articles elsewhere that cover theme creation though
Don’t forget to add the wp_head and wp_footer calls in though, else you won’t get the admin toolbar when logged in