Throw in a redis client or similar and you could have multiple players battling it out. (Assuming you don't want to put stuff into the actual folder on a per-player basis)
A local, cheaper alternative is to just have different player profiles (with different data files) and make them battle when they both happen to be on the same directory.
This would allow an event to be generated when players are in the same folder; allow them to be aware of each other; allow interaction, eg they could draw swords; or interact with the creatures therein. Objects dropped could be seen by both and once picked up are taken by one player etc. Monsters could wander around as well.
I'm not referring to messaging / events for just chat purposes even though you could naturally enough do that as well.