Excerpt from Alon’s blog:

Download the patched version: termsrv_sp3_patch.rar (UPDATE: sadly, I have unlinked this since it no longer works and since this is so many years ago, I don’t think I have this file anymore either. Maybe this article will still help if you can find that file online somewhere)

Installation instructions
0. Go to C:\Windows\system32\dllcache and rename the termsrv.dll to termsrv.dll.bak
1. Go to \Windows\System32 folder and rename termsrv.dll to termsrv.dll.bak
2. Copy the new SP3 patched termsrv.dll to \Windows\System32 folder
3. Run the included registry patch ts_concurrent_session_patch.reg (by double clicking it), it will update the relevant registry values which are relevant to the concurrent sessions support.
3. Restart windows.

if you are currently using Terminal Services, you will need to start windows in safe mode before you can overwrite the existing termsrv.dll files.