Check the timings of those two ram. They have to match to have the same speed. if one is cl 19 and one is 18. it will default to the slowest one. Dropping in the same speed ram with a different brand doesn't mean they will perform the same. Different manufactures have different timing/speed modes.
...you need to return that stick of RAM and get the regular Crucial 2666 CL19 if you want to run dual channel 2666.
That stick you have has mismatched timings compared to the stock onboard, and the stock can’t or won’t handle CL18 at 2666 so the system automatically downclocks to the next JEDEC timing that can run CL18... which happens to be at 2400, er go your dual channel 2400 setup you have now.
This is what I’ve used in my GU502GV and what I’ve recommended many times to others for guaranteed dual channel 2666 compatibility: