Two identical memory modules with different density (for example, 8Gb and 16Gb) with the same Part Number does not boot up the NUC
When inserting either stick of memory, 8Gb or 16Gb, the system boots up normally
When using two memory modules of the same density simultaneously, system boots up normally
Definition of Terms:
Module capacity: The memory capacity of the memory module is usually measured in gigabytes (GB).
Memory density: the memory capacity per chip, usually measured in gigabits (Gb). We also call this characteristic memory technology.
Resolution
It is advisable to buy a memory kit when using dual configuration. When buying a memory kit, it is expected that both modules will have the same specs compared to buying two separate modules, whichhas a risk of having different densities, timings, etc.
Most non-kit combinations work, however, they are vulnerable to the following issues:
Higher risks of compatibility issues
When you use a non-kit combination memory controller, it will try to synchronize the memory module; hence, both modules will work at the frequency and timing of the slowest one.