It's difficult not to sound like a broken record, but
Please don't combine memory kits and ask why XMP will not work.This is a historical issue enthusiast face that will probably continue. However, the introduction of DDR5 has brought in far more unsuspecting users seeking answers.
The main culprit is the lack of 4 DIMM kits currently on the market for DDR5. Most users where possible do tend to buy kits at the rated density, timings, and frequency. However, if you remove those kits from the market you leave a gaping void to be filled with users looking for 64GB and/or simply to populate all available memory slots.
The reason you should avoid combining kits is actually painfully simple, however, the whys and buts are best left to actual DRAM and electrical engineers.
The memory kit is binned by the vendor to operate at XMP in the exact density it is sold in. If you combine them, you will need to consider the following things, and there will be only so much guidance others can give you.
1). Evaluate if the kit is able to run at or even close to the rated frequency anymore. This depends on the amount of guardband the memory vendor has put in, the motherboard, platform and CPU. *
2). Evaluate if the kit is able to run at or close to the primary timings the memory vendor has put in. This also depends on the amount of guardband the memory vendor has put in.
3). Evaluate how much voltage the memory needs for the above two points.
4). Evaluate the motherboard sub timings to see if the kits are able to run at or close to the auto ruling assigned by the board vendor.
5). As with any memory kit, evaluate what voltages are needed on the CPU side. (MCVDD / CPU SA).
Best of luck
9800X3D / 6400 CAS 28 / ROG X870 Crosshair / TUF RTX 4090
