Setting it up (concurrent sessions)
There is a huge difference between Windows Home and Windows Pro/Ultimate. Most of this relates Pro/Uptimate systems. RdpWrap (link below) allows connecting to a concurrent session in Home systems, too.
Once in a while Windows Updates might reset the configuration and you must re-run the setup.
How to shadow in Windows 7?
If you have set up RDP, open Task Manager > Processes tab > Elevate it: click on Show processes from all users > Click on Users tab > Right-click on the user to be shadowed > Remote Control.
You can set the desired access rights in Group Policy > gpedit.msc
Local Computer Policy > Computer Configuration > Administrative Templates > Windows Components > Remote Desktop Services > Remote Desktio Session Host > Connections > Set rules for remote control of Remote Desktop Services user sessions: Enable the configuration and select the desired level of control and permission.
How to shadow in Windows 10?
Shadowing activated from the mstsc command line with options -- check mstsc /help
Configure access rights from Group Policy (see above) or from RDPConf.exe of RdpWrap package.
It takes a lot of typing each time to start shadowing. To make it easy, I put a file shadow.bat in Windows System32 directory with the following instructions:
for /f "tokens=3" %%a in ('qwinsta ^| find "console"') do set id=%%a
mstsc /shadow:%id% /control /noconsentprompt
That allows opening an elevated command prompt and just type as easy as