nobody can tell you.
Since in the first place, using two different sticks of RAM that weren't a part of the same initial kit, are not guaranteed to work together and you mileage may vary.
So it's basically a try it and see if it works kind of thing.
But it's not just RAM speeds matching, but more importantly the timings.
If the timings are not the same, then you'll definitely have problems.
My recommendation is to just pony up the cash for a 16GB (2x8) kit.
Since you motherboard limits to 2400Mhz I offer you this kit as a suggestion.
https://www.newegg.com/Product/Product.aspx?item=N82E16820231888
Sale ends in 3 days. Don't know how much it will cost after sale ends.
But you can use PCPartPicker to filter out 16GB (2x8) kits and organize the list by price and pick out the cheapest kit at the time of your purchase.
However, I should note that I offer this suggested RAM based on the assumption you live in America.
If not in America, tell me what country you are in so that I could better provide an option.