You need to update .htaccess for wordpress.

Here are the content and explanation of new .htaccess.
1. To make Trac and wordpress work together, we need to add
RewriteCond %{REQUEST_URI} !^/trac*
inside wordpress rewrite rules

2. To make Trac HTTP authentication work, we need to add
ErrorDocument 401 /401.html
and create 401.html on the same directory as wordpress.
Taken from: http://wordpress.org/support/topic/htaccess-and-subdirectories

401.html can be empty file or use the following HTML tags.

Authorization Required

It will only display when someone failed to login.

ErrorDocument 401 /401.html
# BEGIN WordPress

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_URI} !^/trac*
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]

#END