Not old-school by choice, but there wasn’t an MSM when I set the sites up and the way they are setup won’t work well with the MSM. 😊
In any case, from one EE install I run two sites:
http://www.westeros.org/
http://www.hippoiathanatoi.com/
I also have five subsites on westeros.org, each with their own path.php, but they’re not an issue as far as logins go since the cookie gets set for all of westeros.org (they’re not subdomains, just folders, like http://www.westeros.org/GoT/ and so on). So the login issue is just between the two different domains. Here’s the path.php for Westeros:
<?php
// DO NOT ALTER THIS FILE UNLESS YOU HAVE A REASON TO
// Path to the directory containing your backend files
$system_path = "/home/sites/westeros.org/www/html/Scripts/ExpressionEngine/SYSTEMDIRECTORY";
$template_group = "";
$template = "";
$site_url = "http://www.westeros.org/";
$site_index = "";
$site_404 = "";
?>
It is exactly the same for the other site except the site url is http://www.hippoiathanatoi.com/.
Ultimately, though, I don’t think the path.php is the issue here. I think it is with the config.php option I initially mentioned:
$conf['multi_login_sites'] = "http://www.westeros.org/|http://www.hippoiathanatoi.com/";
Before I upgraded, each login would log me into both sites. But now it just logs me into the first domain. Surely it has to relate to the changelog item I quoted? (“Fixed a bug (#17216) where the “Allow multiple log-ins” config was not enforced strictly enough.”)