Give this a shot, yo. I did this in WP 2.7.1.
Once you have the users sharing a database table as you described above, open your wp-config.php for both blogs.
Copy the AUTH_KEY and SECURE_AUTH_KEY lines from your main blog's wp-config.php to your second blog's wp-config.php so that they are the same in both.
Create a line in your main blog's wp-config.php file that looks like this:
You should replace the salt value string above with your own, possibly generated for you using this page. DO NOT simply copy and paste the above line, because that would be wrong-hearted.
Once you've done that, copy the SECRET_SALT line from your main blog's wp-config.php to your secondary blog's wp-config.php file so that it is the same in both.
Create the following lines in both wp-config.php files:
define('COOKIEHASH', md5('Another random string value!'));
- COOKIEHASH is normally defined in your wp-settings.php file on line 351. Unlike most of the other constants here, it doesn't check if it's already defined before defining it. On my server, this doesn't cause any problems, but if you notice error messages like 'COOKIEHASH is already defined' or something, that's probably the problem. If you want to hack up core files (which we've already done by now), you can comment out that line in wp-settings.php and that should take care of it.
- Remember to replace the COOKIEHASH value above with your own string.
Save both wp-config.php files, clear your browser's cookies and log in to either blog. Once your in the admin area, try hitting the admin area of the other blog and it should work fine.