I installed the first one (called A) as a normal WordPress.
When I was loggued on the admin of A, I modified my wp-config.php to add this line :
So, the Tools/Networks appeared.
So, I choose the "Subdomains" option (tutorials say you need to choose it) and I validated.
After, I made the needed changes on wp-config.php and on .htaccess.
But, when I try to reconnect, I cannot, the login form doesn't recognize me (if I delete the added lines on wp-config.php, it works).
Can you help me or give me an url for an up-to-date tutorial ?