A single-SIM variant S9 plus will only work with one SIM, that's it. A dual-SIM tray will physically fit the single-SIM phone, but there's just nothing inside the phone to connect and use the second SIM. The SIM2 position doubles as the SD slot, but in the single-SIM variant it's SD only.
FYI dual-SIM variant S9 pluses are usually sold and used in Asia, and they're available in Europe as well, and always unlocked.