Re: x299 dark and mixed speed modules quad or dual
Unfortunately, this wouldn't be a recommended configuration. If you must use these Intel only certifies the speed of the memory controller up to 2666/2400MHz for Kaby Lake-X and Skylake-X platforms, respectively, and all speeds above Intel’s certified speeds require overclocking. This means, it'll most likely run at stock speeds, and you'll most likely have to adjust the speeds manually. Which still wouldn't guarantee you'd hit those clock speeds.
I'd personally pick up a quad channel stick if you'd need to and sell the two dual channel kits.