Dual-channel still works with soldered + replaceable RAM — usually.

Yes, but only if the soldered stick and the replaceable stick are the same capacity and speed. Even then, it’s not always full dual-channel across the whole memory space.

Most modern laptops use something called “flex mode.” If you have a 4GB soldered stick and add a 4GB replaceable stick, the first 4GB of each will run in dual-channel, and the remaining 4GB from the larger stick (if mismatched) runs single-channel. Same capacity? You get full dual-channel. Different capacities? You get partial dual-channel.

The real gotcha is that you can’t always control the soldered stick’s specs. It’s whatever the manufacturer decided. So check the system’s manual (or a tool like CPU-Z) to see the soldered RAM’s speed and timings. Match those exactly with your new stick for best results.

Bottom line: It works, but you’re at the mercy of the laptop manufacturer’s soldered choice. If you can, buy a laptop with socketed RAM from the start.