Ram is made up of IC chips soldered to a pcb. Each chip is cut from a sheet of silicon. Each sheet of silicon has different levels of impurities. As the sheet is spun out, some of the heavier impurities tend to gather towards the outside.
That said, when the factory makes the ic's and sticks them on the ram, it decides what kind of kit it'll go in. 1 stick, 2, 4, or 8. It'll test these sticks for compatability, replacing any that don't comply, so when you buy a kit, it's guaranteed to be compatible, every stick.
Here comes you with 2x kits. Factory did not test those 2, so no guarantees on compatability. They come from different batches, made from different silicon sheets, with a mix of impurities. Further increasing the odds of incompatibility. 2 different sizes means different amounts of ic's in some way, could be 512Mb or 1Gb or even 2Gb ic's or all the same just double the IC counts. Which again increases the odds of incompatibility.
When Darkbreeze said its a roll of the dice, it's exactly that. There's only one guarantee about mixing kits, there are no guarantees on anything. They may play nice perfectly, they may need your input and changes in bios to play nice, or may not play together at all.
I've mixed corsair ram with Kingston, different speeds, voltages, sizes, models, timings and had them work perfectly with no changes by me. I've had 2 kits of exactly the same corsair ram, bought on the same day, from the same store, same shelf, 1 kit right after the other, and the actual manufacturing numbers were only 9 sticks apart and absolutely would not work.
There's no rhyme or reason to it, ram is ram and will do as it pleases, the only way you'll ever know if it will work, is to try it personally, you become the tester.
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