http://h20628.www2.hp.com/km-ext/kmcsdirect/emr_na-c04224518-1.pdf
Go to Page 90 to see the information on the memory. Look at the System Board diagram and lables to see how the slots are grouped. They are grouped in one white and one black in Channel A and the same in Channel B. So, you don't use both whites or both blacks, but one white and one black in each Channel.