This question was posted in response to the following article: http://help.adobe.com/en_US/ColdFusion/10.0/CFMLRef/WSc3ff6d0ea77859461172e0811cbec22c24-7 c5c.html
Currently, CF10 disallows concurrent logins. Previous versions of CF allowed concurrent logins. For a temporary workaround, please see John Jarrard's note here: http://blogs.coldfusion.com/post.cfm/new-improved-cflogin
Not only can you not have 2 concurrent logins into a single coldfusion application, you can't have logins into 2 separate coldfusion applications that happen to share the same domain. This seems to be an undocumented change since CF10. If you are logged into www.example.com/app1 and log into www.example.com/app2, coldfusion will log you out of app1.