Mixing voltage should be fine, but you do need to make sure that the memory types don't mix. If both are RDIMMs, you should be fine.
Hi i'm planning to upgrade ram in this server. It have one RDIMM in A1 slot type of ram 8GB EEC Dual rank ddr3l (1.35v) registered 1333Mhz.   My question is can i add 1.5v memmory together? For an example an another RDIMM to A2 slot? Thanks in advanced. 
Memmory upgrade r320