They can work together. The slot stays active.

Yes, you can use both. The soldered RAM does not disable the SO-DIMM slot. Most laptops with this design simply let you add more memory in the empty slot, and it will run alongside the soldered chips.

The only catch is speed matching. If the soldered RAM is, say, 3200 MHz and you put in a 2666 MHz stick, both will drop to 2666 MHz. Dual-channel will still work if the capacities are right, but you might lose a bit of performance if they’re mismatched. Some laptops also force single-channel mode if the sizes don’t match