using xcopy and a loop i found this is a good way to make sure that changes in one file make their way across to another location using a windows batch file. this one loops every 5 seconds, i think you could run it without the 5 second timeout but it might cause excess cpu and drive usage.

:loop
REM xcopy   /D /Y
REM this will copy the file only if its changed from source to destination and check every 5 seconds for a change
xcopy file.txt file_backup.txt /D /Y
timeout /t 5
goto loop