A suitable post build step would be this:
if NOT $(ConfigurationName)==Debug goto done
del $(ProjectName).debug.exe
copy $(TargetFileName) $(ProjectName).Debug.exe
:done
(Assuming $(ProjectName) is the same as the executable name without the .exe suffix.)
It doesn't need to be in a batch file; paste it straight in to the post build settings.
Originally, instead of the copy I was just renaming the .exe like so:
ren $(TargetFileName) $(ProjectName).Debug.exe
But that had a really annoying drawback - when you go to debug, it says the .exe is missing (which it is of course). So I ended up just making a copy.