I have a blog, lets say example.com and another blog installed in example.com/np which is not multisite but a different WordPress installation.
You don't want to muddle in your Wordpress templates or code for this, just use a single mod_rewrite rule in your .htaccess:
RewriteRule / /np [R=301,L]
Put it below the RewriteEngine on line if it's already there, else add it as a separate line above the RewriteRule line.
This solution is easily removable, easily maintainable, portable, and performs better than doing it in PHP in the templates or WP code, and survives updates to your templates.