your 1.5vDDR3 is likely stock speed stuff while the 1.65v is likely Overclock or higher speed ram. Your system will only run all the ram at the speed of the slowest ram installed so while you may be able to run with both types installed - you will be running at the slower speed.
If you wanted to proceed I would try the 1.65v ram at 1.5v and slowly increse the ram voltage until the system was stable. You do run the risk of burning out the stock 1.5v ram. How big a risk depends on what ram it is.