You might be able to get away with it assuming you are talking about DDR3 and they are the same CL and voltage rating. Different speed is not so important as the motherboard memory controller will only run different memory speeds at the slower speed (meaning it will run your 1866MHz stick at 1600MHz).
DDR4 memory is a lot more finicky to mix and match. There's only one way to know for certain: try it. You won't harm anything. You either will just get a crash, failed boot, or the second memory stick won't be recognized.