Dual-channel works with soldered + replaceable RAM — if they match.

Yes, but only if the sizes and speeds match.

Most modern laptops that have one soldered stick and one slot support “flex mode.” That means if the two sticks have the same capacity and speed, they’ll run in full dual-channel. If they don’t match — say 8GB soldered + 16GB replaceable — only the first 8GB of each runs in dual-channel; the rest is single-channel. Same for mismatched speeds: the faster stick slows down to match the slower one.

In practice, matching capacity is what matters most. Speed mismatches are less noticeable. And some laptops are picky — check your manufacturer’s docs before buying.

It’s not as fast as two identical sticks in matched slots, but it’s way better than leaving the slot empty.