...
mkdir x64
mkdir x64\Release
nmake /f win32\Makefile.msc
copy zlib1.* x64\Release
nmake /f win32\Makefile.msc clean
...
Now, the 64-bit Debug build, switching back to the x64 Tools command prompt:
mkdir x64\Debug
nmake /f win32\Makefile.msc
copy zlib1D.* x64\Debug
nmake /f win32\Makefile.msc clean