We have defined a splitting value gathered from server variable attached to all requests. We use this value in a business transaction to group purepaths by user.
I have two questions related to this splitting value:
If you would have UEM (User Experience Management) then you would automatically get Concurrent Users reported as dynaTrace UEM takes care of identifiying individual Visits. It might be a bit hard to figure out the "Concurrent USers" with your approach - But - here is a different thought: Typically AppServers (ASP.NET, JBoss, Tomcat, ...) provide performance counters for "concurrent sessions". Have you looked into that possibility?
If you continue with your approach then I can tell you that dynaTrace has an auto purge feature in the performance warehouse where measures from split values that didnt produce values in a while get automatically purged. For more details search for "auto purge" in the following doc page: Performance Warehouse Administration