It is normal to have two instances of csrss.exe running. Both should have a system32 directory source.
Note: There is a command line associated with this process. On my Win 7 x64 installlation, the command line for both is:
%SystemRoot%\system32\csrss.exe ObjectDirectory=\Windows SharedSection=1024,20480,768 Windows=On SubSystemType=Windows ServerDll=basesrv,1 ServerDll=winsrv:UserServerDllInitialization,3 ServerDll=winsrv:ConServerDllInitialization,2 ServerDll=sxssrv,4 ProfileControl=Off MaxRequestThreads=16
You can download ProcessExplorer from the SysInternals web site which will allow you to view the above detail.