you can only use them together if only they support auto-mode with somewhere same voltage/timing/frequency settings.
but fyi information, they will be running with the slowest setup coinciding setup between them.
what I am trying to say is that for instance many XMP profile memories supports also normal scenarios as long as motherboard allows them.
so eventually both memory sets should have same setup somehow to run together.
but personally I wouldnt mix different voltage memories in any case...