Depends on the CAS latency of the RAMs. They will certainly work together fine but at the fastest speed that is compatible to both sticks.
Say one of your sticks supports C16 and another C18 @ 3000Mhz then the C16 stick will work on C18 only as the stick which supports max C18 cannot go any faster hence the faster stick has to work slower.
It's not a game changer difference so no need to spend any extra, just use as-is.