Many individuals use the Event Log to monitor the number of users.
I had a similar issue to what your asking. Our management wanted to have a database that maintained a list of users currently logged on, time/date of logon, time/date of logoff, machine name, IP address and some other information that we could gather.
What I did was to setup a Clustered Microsoft SQL 6.5 Server (using Microsoft Cluster Server 1.0) and a shared RAID Array. We did this to have a relatively high level of service. Next, I created a Visual Basic 6.0 application (you can use any language/scripting language you want) that would be kicked off via login script. This executable would collect the necessary information and update the SQL Database accordingly.
As a result of this effort, we developed scripts to automatically delete user accounts that had not logged in for 6 months and various reports that the business requested. In addition, we had an email sent to the user if they had 2 or more machines logged on at the same time - asking them to please logoff if the extra machines were not in use (for security reasons).
If all you need is a list of users, login times and machine names, you can do that quite easily with most scripting languages. A free language KIXTART (NT Resource Kit) is decent. You can also try WSH, VBScript, JavaScript or WinBatch if you are more familiar with those.
One thing that we really need to know is whether or not you intend to kick the user off (deny login) immediately from the 2nd system or is this just for reporting purposes?
I would be more than happy to help you out with this project. Just write to me Consultant@ShaneHage.com