The answer to my question is the element. To see it in the GUI, bring up project properties, choose "C++", and then "General". If you set it to true, then that project will first build the PCH file, then it will spin up a "cl.exe" process for each c/cpp file up to the number of processors the machine has. Once all files are built, then it will link with one process.
This is different than the fact that Visual Studio will also spin up multiple copies of msbuild to build multiple projects at once.