I have two different ram sticks with different frequencies. As of right now I have 16gb DDR4@ 2400 MHz in my system. I was wondering if I could also add an 8gb DDR4 @ 2133Mhz in my system.
Adding or mixing memory is hit and miss. Even identical modules purchased at separate times can be incompatible. You won't know till you try them. If they do run they will run at 2133.
Memory is guaranteed in the form sold. You can make any combinations you want. It's just that there will be no guarantee they will be compatible together.