Ultimately, he realized it was that when one of his colleagues logged into CF 10 Admin, he got logged out, and vice-versa. Certainly frustrating.
And yes, it's by design in CF10, as part of various security enhancements.
Where's it documented?
It is documented, in a couple of places.
Update: Since I wrote this entry back in June '12, I did a video for Adobe about a year later where I walk through this in several minutes. You may want to check that out.
Her article notes with respect to changes about CFLOGIN, which the CF Admin uses under the covers:
Now you can have only one active session open for one user for a given application that uses the cflogin tag.
For example, you can now access the Administrator console one user at a time with a given set of UserIDs and passwords.
Now, you may think, "but that doesn't explain why another user and I can't use the CF Admin at the same time", but actually it does. Note that the login is not "per user" but "per account" using the CFLOGIN. And by default, there is one account used for logging into the CF Admin, called the Admin user. We normally don't even notice or use that, and only need to enter the password for that account.
So it's saying that "2 users of the same account can't be logged into the CF Admin at one time." I agree it's an annoyance, but I'm sure there's a worthy security problem for which it was the solution. Maybe someone from Adobe will chime in with more thoughts.
There's also a discussion related to "logins to the CF Admin" in the CF10 docs, in the "Developing Coldfusion 10 Applications" manual, though it's not worded as applying so obviously to this specific situation above. At the bottom of this page on Miscellaneous CF10 changes, it says:
You are logged out from one of the ColdFusion administrators, if:
From the same host, you log in to the ColdFusion (10) Administrator and the ColdFusion Administrator of an older version.
A solution that may help
Here's something that may help. Many never noticed but CF8 added the ability (Security>User Manager) where you could define additional username/password combinations in the CF Admin (including limiting what parts they can access, including the Server monitor) and also for controlling RDS access.
This feature may be all the more valuable now as one way to solve this problem of multiple users needing to be logged into the CF Admin at once. I did a fairly extensive article on the feature feature from 2009 in the Adobe Dev Center. At the time I wrote that (CF8 timeframe), the ability to add admin users was limited to CF Enterprise, but in CF 9 that was lifted and available also in Standard.
Hope that's helpful to someone. And perhaps if there may be more discussion on this topic (of the forced log off in CF10 of users sharing a single Admin account), this can provide a place for it.
Update: As another update, besides noting that you can see my later video walking you through things, Adobe said at the CF Summit that they would be lifting this limitation of a single login per account in CF 11, configurable through an Admin setting.