Memory manufacturers guarantee the rated speed and timings of any memory to be obtainable only in the kit it is sold in. If you are adding an additional set of memory, the rated timings may not work due to the extra stress on the memory controller. Therefore, you will need to relax the timings and/or frequencies in order to get two sets of memory to work. Another option you may need to do is increase the dram voltages or reducing the CPU multiplier.