1 stick of ram in package. Factory tested for working.
2 sticks of ram in package. Factory tested for working and working together.
4 sticks of ram in package. Factory tested for working and working together.
Nowhere in that is factory tested for seperate kits working together. It doesn't matter if same brand, same model, same color, speed, timings or otherwise identical ram kits, they are not factory tested for working together. So that makes you the tester. They may work, they may need adjustments to work, they may not work.
The only guarantee about mixing kits is there is no guarantee. It's totally pot-luck.