, cookie_value, cookie_expiration, path
If both sites are on localhost, then there is no domain
You probably cannot change the paths, most software sets cookies with the path of / so there is no path
I looked over a demo and there was no way to change the cookie name
s so you cannot separate them like that.
So, both instances have the same domain, same path, same cookie names. Basically the second log in overwrites the cookies set by the first login.
Edited to add:
And even if you used same username and password on both, there is most likely a session cookie as well and like the others gets overwritten too. When you go back to the first the session will not match and it will kick you to the login.
All is not hopeless however.
Use your windows hosts file and add an entry for each .... say for example forum1 and forum2
Give each one a VirtualHost in Apache and use the ServerNames of forum1 and forum2. Set the DocumentRoots of each virtualhost to their respective locations for example c:/xampp/htdocs/forum1 & c:/xampp/htdocs/forum1
Give them the least needed permissions for it to run. Being that the docroots are under htdocs (assumption here) you probably do not have to worry about this as they will pick up the same permissions as c:/xampp/htdocs.http://forum1http://forum2
Now you have them separated them by domain and they cannot overwrite each other.