To enable concurrent remote desktop session, Go to
Open Control Panel & Open Administrative Tools
Go to Services
Choose Remote Desktop Session then Right Click & Stop the Services.
Goto c:\windows\system32 & copy termsrv.dll and paste it in the same folder. It automatically paste the copy of termsrv.dll.
Right click termsrsv.dll -> security -> system ->advance-> change-> administrators. Now administrator user get full access to change/modify termsrv.dll file in system32 folder.