Yes, given that both of the RAM sticks are of the same type (DDR4 or DDR3). But when the 2400MHz RAM stick is paired with the 2133MHz RAM stick, both of the RAMs will run in dual channel configuration @2133MHz. read more
You can use them together but they'll function on the lower speed of the mix, ie. 2400 and 2133 with be like two 4 gig sticks at 2133. You can manually overclock your sticks if you want. Get a 2133 stick if you have a 2133 stick already and save some money. read more