Probably, but you might lose dual-channel on the 8GB portion.

Yes, it will almost certainly boot and run at 2133MHz. Most modern motherboards handle mixed-size sticks fine. But there’s a catch: dual-channel memory only works in matching pairs.

With a single 8GB and a single 16GB, they’ll run in what’s called “flex mode.” The system treats the first 16GB (8+8 from each stick) as dual-channel, and the remaining 8GB on the 16GB stick as single-channel. In practice, you probably won’t notice the difference unless you’re doing memory-heavy tasks like video editing or gaming at high framerates.

The bigger risk: if your motherboard is picky about single-sided vs. dual-sided sticks, or if the timings are slightly different, it might not POST. Most 2133MHz sticks are DDR4 with loose timings, so compatibility is usually good. Check your motherboard’s QVL list if you want to be paranoid.

I’d say go for it. Worst case, it doesn’t work, and you return the stick. Best case, you have 24GB for cheap.

Variance is the enemy of certainty here. Just make sure both sticks are the same voltage (1.2V for standard DDR4).