Preexo,  I am assuming this is a single processor configuration? If not, then let me know. If this is a single processor configuration, then you can place the 16gb dimms in the white tabbed slots A1 and A2. Then place the 8GB dimms in the black tabbed slots A4 and A5. In the BIOS, under Memory Settings, make sure it is set to Optimizer Mode. That will be a supported configuration, as long as they are compatible 16GB dimms.  You find more information on the system memory here. Let me know how it goes. 
Hi, we have a poweredge T320 and want to extend the memory of the existing 2x8GB.  What should the configuration be like if we want to add 2x16GB?  Thanks in advance for the help and best regards Tim
Poweredge T320 mixing RAM 2x8GB + 2x16GB