In the Tomb phase, each creature will move towards/attack the closest adventurer. If multiple adventurers are equally close, the adventurer with the most soul shards is attacked. So yes, one adventurer will get ganged up on. Plan your actions carefully.
All tomb-phase combat is considered to be simultaneous, so the other adventurer in the room isn't sitting around writing a novel.
Thanks for answering Matt!
Let me know if you need anything Jason!