You can install 4 x 4gb RDIMM's in slots A4,A1, B4, B1, which is channel 0. You can put 4 x 8gb RDIMM's in slots A2,A3,B2,B3 and set to Optimizer Mode. Optimizer allows all 4 slots to be used, but will not use SDDC (Single Device Data Correction). This will give you 48gb of RAM.
Each channel on both processors has to be identical. Channel 0 is A4,A1,B4,B1, Channel 1 is A2,B2, and channel 2 is A3,B3.
Hope this answers your question..