For dual CPY R740XD2 config you can use all 16 RAM slots. You can refer below link for details [URL] Below is an extract from above link  
Hello all, I am just trying to get some clarification.  This server provides 10 RAM slots for CPU 1 and only 6 RAM slots for CPU 2.  With two processors installed can I only use 12 RAM slots, 6 for each CPU.  Or can I populate all 16 slots.  I am thinking that using the full 10 slots for CPU 1 is only for a single processor install.  Thanks, Dave C
r740xd2 ram population