Good find! To be honest I haven't tested a lot with multiple player characters so am not too surprised. You say you discovered this by looking through the code. What specifically tipped you off?
Hey, awesome to see you read these comments! I've been enjoying spending a lot of time just reading/rewriting/mucking around with the code and seeing ways this project could be taken further. In this case, it seems to be due to the merging of states. As you know, states nodes are created, and then compared to existing states, to see if it's actually an already existing one. In the case of 2 players (other than when they can't reach each other), every state (other than when they are in the same node), involve an exact replica, in which the two players are switched. This visual bug is caused due to the state in which the players are where you'd expect, having been replaced with the state in which they are switched. The ideal solution in my mind is not so tightly linking specific 'movables' to each state, but instead merely linking the concept (or definition) of that movable to the state, and then evaluating which particular movable is most appropriate to animate to the new location.