Intel Flex Mode handles mismatched dual-channel RAM.

Yes, most modern laptops with soldered RAM and an open slot can run dual-channel even with mismatched sizes. Intel calls this Flex Mode; AMD has an equivalent. The overlapping portion (e.g., 4GB of an 8GB soldered stick plus 4GB of a 16GB slot stick) runs in dual-channel, and the rest runs single-channel. You don’t need to worry about “same bus” — the memory controller handles it.

To check if your specific model supports it, look up the CPU generation. Intel’s Flex Mode has been around since at least Haswell (4th gen). For AMD, Ryzen mobile chips support asymmetric dual-channel. If you’re paranoid, find the laptop’s service manual or run CPU-Z after installing the extra stick — it will show “Dual” in the Memory tab