Usually, but it's a gamble.

Yes, your motherboard will try to run both sticks together, but it’ll slow both down to match the slower stick’s timings. If the timings are close, you might not notice. If they’re far apart, you could get crashes or boot failures.

The real risk isn’t the timings—it’s the underlying memory chips. Two sticks from different batches or brands might just not play nice together. I’ve seen it work fine for years and I’ve seen it refuse to boot.

If you already have the stick, try it. If it’s stable after a few days of normal use (including games or heavy tasks), you’re golden. If not, return it and get an identical match or a kit.

Future You deserves better than random kernel panics.