If you mix different ram modules, you will only be able to run at the speed of the slower module (so both will work at DDR 4 2400) however it should work. That said, different ram modules don't always play nicely together, ideally if you want to add another module you would look for an equivalent stick to match the one you already have.
As I say - it will probably work (especially as DDR4 2400 is the base speed for DDR4 memory so pretty much all modules should work at that speed).
Just to highlight even buying the exact same make and model RAM is not guaranteed to work. There is a good chance it will work but not guaranteed. I have once found 2 Corsair kits of same model that would not play nice in any combination. Ultimately only RAM bought in a kit is guaranteed to work together.