Soldered 8GB + added 8GB vs full dual-channel: Flex mode is the likely outcome

Adding 8GB to soldered 8GB probably won’t give full dual-channel. You’ll likely get flex mode (partial dual-channel) or just single-channel, depending on the laptop. Most laptops with one removable slot and soldered memory treat that slot as a single channel, so the first 8GB (matched portion) runs in dual-channel while the rest runs single-channel.