1 login, 2 WordPress sites, subdirectories

Hello,

I know this has been asked a lot but most of the answers/posts i find don’t apply to my case (i think).

I have a website wich uses Magento in its root and I want to have a blog, forum and Q&A section.

I have installed WordPress #1 in http://www.site.com/blog (uses a modified theme)
For the forum I used the BBPress plugin but for the Q&A I want to use AskIt (a theme by elegantthemes).

Because AskIt is a theme by itself, I have installed WordPress #2 on http://www.site.com/QandA.

Now I want both WordPress sites to use the same login.
How do i do this?

I found this link http://kiranthomman.com/setting-single-login-across-multiple-wordpress-27-installs-using-a-single-domain/ but as soon as i change the config and capabilities files i get a not enough rights error. (trying to log in as admin)

and

I also looked into the Network option in WordPress itself but it states

You cannot create a network in the following cases:
“WordPress address (URL)” is different from “Site address (URL)”.

wich is my case.

Please help