Wow, this was a problem and very frustrating. I offer this information without warranty, but this is how I got Windows 8.1 Pro to allow muli user logging so that multi users could access Capital simultaneously.
1. Windows 8 fix is not the same as Windows 8.1. The Windows 8.1 update, killed the Windows 8 fix.
2. Obtain and run the Windows 8.1 Terminal Services Patch Utility. This amends the registry, but installs a broken termsrv.dll driver.
3. Hunt the internet until you find a patched Termsrv.dll for windows 8.1 specifically. Should be 995kb.
4. Basically replace the \windows\system32 driver with the patched 8.1 driver.
To avoid any further frustration from windows automatic updates (and cause staff downtime), I scheduled a batch file to copy the driver on a daily basis.
Hope this can save someone the many hours I lost looking for a solution.