DDR 3 voltage mix

Had a friend ask me a question. Did not know what to tell him...

He has 8GB of DDR3 memory (2x4GB sticks) running at 1600Mhz and 1.65v

He wants to know if he could add another 8GB (2x4GB sticks) of 1600Mhz DDR3 that runs at 1.5v

Mobo is MSI Z77A-G43.

1333 1.5v and see what happens, The board would probably default them to that anyway, from there he might be able to get to the middle ground and be stable depending on the sticks. There is no definite answer, depends on the sticks.