My experience comes from an Ivy Bridge machine with a B75 chipset and 4 RAM slots, YMMV.

I had a single 2GB 1333MHz stick and bought another one second-hand. The sticks had different densities (one had 4 and the other 8 memory chips on) but timings were the same. The system would not boot with the sticks on the same bank (same slot color) but when I moved one stick to a different bank, the system booted and BIOS indicated the memory was operating in dual channel mode at correct timings. I didn't experience any issues with the system for 1 year.

If your machine only has two RAM slots with a single bank, then it's a crapshoot on whether the system will boot. With two different banks you can usually mix non-matching memory.