Take in consideration that Ram is sold in kits that have been tested to work together to avoid issues you may encounter with mismatched ram. Also memory manufacturers and vendors won't provide support for that configuration.
- If both sticks work, but they run @ 2133-2400MHz you could raise the speed in increments manually until you find the top speed that they could work. You would have to set the timings manually.
-Some motherboards can be very picky about RAM and it might not even boot with mismatched RAM.
-Sometimes as you mentioned RAM might default to 2133 or 2400 MHz
-Other times Windows might only be able to use one RAM stick.