I am having quite the frustrating issue right now. We have one install of WP in a directory called /updates/ that is working great.
I have recently added a new installation of WP on the same site at the /news/ directory.
When I login to the admin panel of one, I basically can't login to the admin panel of the other. Once I hit submit, it redirects me to the login page of the other blog and says "incorrect user name/pass".
If I managed to get logged into both, it will usually kick me out of one and redirect me to the login page of the other.
I have checked htaccess at the root level, and on both installs and there is just nothing that should be blatantly redirecting like this.
Where are some other places I could be looking to find the source of this trouble?