Ideally you should use 2 sticks of RAM that were purchased as a matched pair or at least match each other as closely as possible.
It will depend a little bit on exactly what RAM and motherboard you are using but in some scenarios it is possible that what you have COULD work. You'd need to have all the RAM settings match the "worst case" spec for each stick of RAM. Also, even if that combination of RAM boots and is stable, it is not a recommended configuration.
Specifically you'd want to chose the slowest timings from the slower stick, but you'd also likely want to chose the voltage from the higher voltage stick. NOTE: Running 1.65V RAM at 1.8V may burn it out quickly or can at least decrease its lifespan.
Thought process...