Try looking through the forums, people mixing the same model of DRAM from different packages and it happens all the time, you can pull sticks right off the assembly line and they might not play. DRAM in a package is tested to play together. Also if you already have a stick and now get another, the new may be completely different memory ICs (chips), or even if same they may be from different production lots, binned different, even a change in solder or PCB can affect the electrical conductivity. I'm not saying they WON'T play, but letting you know at best it's a 50/50 crapshoot