This cannot be done with CMake. You have to generate two separate build folders. One for the x86 NMake build and one for the x64 NMake build. You cannot generate a single Visual Studio project covering both architectures with CMake, either.
To build Visual Studio projects from the command line for both 32-bit and 64-bit without starting a Visual Studio command prompt, use the regular Visual Studio generators.
For CMake 3.13 or newer, run the following commands:
cmake -G "Visual Studio 16 2019" -A Win32 -S \path_to_source\ -B "build32"
cmake -G "Visual Studio 16 2019" -A x64 -S \path_to_source\ -B "build64"
cmake --build build32 --config Release
cmake --build build64 --config Release
For earlier versions of CMake, run the following commands:
mkdir build32 & pushd build32
cmake -G "Visual Studio 15 2017" \path_to_source\
popd
mkdir build64 & pushd build64
cmake -G "Visual Studio 15 2017 Win64" \path_to_source\
popd
cmake --build build32 --config Release
cmake --build build64 --config Release
CMake generated projects that use one of the Visual Studio generators can be built from the command line with using the option --build followed by the build directory. The --config option specifies the build configuration.