What do you mean by "dont merge"? you mean they can't run together or you can't do duel channel? etc... what do you mean exactly?
Normally if the sticks are compatible with the board it would down clock the higher memory to the lowest memory speeds and function. You may not be able to do duel channel due to compatibility issues between the chipsets or the board with mismatched memory. If that's the case, you should buy two of the same sticks and to be even safer. Just buy a kit instead.