Multiple Simultaneous RDP Sessions in Windows 10
Option 1 - Modifying termsrv.dll
To have concurrent user sessions working in Windows 10, you need to make small changes to termsrv.dll file. This file is located in C:\Windows\System32 folder. Before modifying termsrv.dll for the first time, you need to take ownership and assign yourself full permissions. You also need to stop Remote Desktop service (TermService) if it's running. To do that type the following commands into a "Run As Administrator" command prompt.
net stop "Remote Desktop Services"
takeown /F termsrv.dll
icacls termsrv.dll /grant:r User:F
The same DLL file is also present in a cache location, I recommend that this file is deleted but can be patched also.
takeown /F C:\Windows\WinSxS\...
DLL files can be modified using any HEX Editor (for example Tiny Hexer). You can do this yourself by replacing strings shown below.
Windows 10 x64 v2004 - May 2020 Update (20H1)
termsrv.dll is version 10.0.19041.84, find the following set of bytes:
39 81 3C 06 00 00 0F 84 D9 51 01 00
and replace them with:
B8 00 01 00 00 89 81 38 06 00 00 90
Windows 10 x64 v1909 - November 2019 Update
termsrv.dll is version 10.0.18362.657, find the following set of bytes:
39 81 3C 06 00 00 0F 84 5D 61 01 00
and replace with:
B8 00 01 00 00 89 81 38 06 00 00 90
References
Not really references, just copied from:
Another interesting article about how to shadow a user's session: