9
Actually it is quite easy without using a helper batch file. You just need to run the application via cmd.exe instead, and make sure to escape the special characters so they pass through to cmd.exe.
You probably don't want to see an extra console window, so use the START /B option.
start /b "" cmd /c myapp.exe ^>myapp.out 2^>^&1
Each STARTed process must have its output directed to a unique file. Multiple processes cannot share the same output file.