Trouble with 2kit ddr3 1866mhz

Hi! i've a problem hope you can help me to solve. I've decided to upgrade a bit my machine, buying another ddr kit @1866mhz. I had already an CMZ8GX3M2A1866C9 memory kit and i've bought another CML8GX3M2A1600C9 sure to be the same memory just with the second one with low profile. If i put a singolar kit on my pc, it work fine with the X.M.P. profile enable, but if i put both kit at the same time the sistem work but only at 1333mhz with the X.M.P. profile disable. I've already try to reset and upgrade the bios but still nothing. It's possibile that this two kit can't work fine together at 1866 mhz? thanks so much and sorry for my english.