Yes, you can absolutely mix RAM speeds in a dual-channel setup. Hell, you can mix it up a bit with RAM capacity or even bring different brands into the fold. That said, there are a few things to consider before you try and spice up those expansion slots with some variety.
RAM is at its finest when the sticks match up, and it’s not like, say, a food or cosmetics company printing copy on their products that reads something along the lines of ‘goes best with our jerk sauce’ or ‘ for best results, use with our face cream’. Identical RAM actually does work better.
Another thing you should bear in mind is that, in a dual-memory system, your computer will run both RAM sticks to the spec of the weaker one. So, if you have one 4GB 2400MHz stick and one 8GB 3200MHz stick, performance will be equivalent to two 4GB 2400MHz sticks.