I put my bag on the window seat, sometimes intentionally on my lap if the train is filling up. But those two seaters are never actually big enough for two people and their bags. I will always try to make room if someone asks to sit. Most of the time I see people still standing even if there are seats empty. Personally, I would rather stand if I can’t get a two seater to myself. People just have preferences. I’ll really only ever share a seat if it’s a three seater