Hi my name is ***** ***** I will be the expert working with you today to help you understand the Dimm 1 & 2 question posted.
Dimm is the memory module or the computer memory stick Dimm 1 & 2 are the slots mentioned on the motherboard of the computer and the memory works on the same Mhz frequency
if 1 module is different from the other memory module the computer will not start or post. It has to be the same Mhz which will be compatible to run.
RAM speed theory (DDR3 1333) (DDR3 1600) will not work it has to be either 1333Mhz or 1600Mhz in pair then it will work.