Here is my code:

# This is for removing extension (Working)
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule (.*) $1.php [L]

# This is my question (Not working, both code has been placed and now showing 500 server error)
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ 404.php [L]