Are both CPUs the exact same model? I've found some info where Device manager lists 8 cores/CPUs that are the same, but you only have four accessible if the second Physical CPU is not the same model.
Are there BIOS updates available for your motherboard? Check and see if there are release notes indicating changes to multi-CPU OS support.
IKt might even be necessary to do a clean Install of Windows 10 in order for both Processors to be available.