There's nothing wrong with your setup - that is how it works. Only one user can be logged in at a time from the same computer. The only exception to this is if you open two different types of browser on the same computer, e.g. IE and Firefox. Then you can be logged in in IE and not logged in in FF, or logged in as two different users in each browser.

The reason that only one user can be logged in at a time from the same type of browser on the same computer is that when a user is logged in, that fact is stored in a cookie file on the local computer. There can only be one cookie per site, but each browser type stores its cookie separately.

Its on a portal (i.e. site) basis. Cookies are stored by address of the site, so you have one cookie for www.abc.com and another for www.xyz.com.

If you are trying to open www.xyz.com and you end up getting redirected to www.abc.com, then that indicates a problem with the portal aliases, i.e. the addresses for each site. Log in as host, go to Host menu > Portals and look at the portal aliases for each portal.