I haven't taken the time to fully understand the write up, but it seems like it would disallow creating a box that encloses both characters together, which woul

I haven't taken the time to fully understand the write up, but it seems like it would disallow creating a box that encloses both characters together, which would be a stupid move, but not necessarily "against the rules". As I understand your algorithm, your true goal is not to determine if cells have a path to the edge, but that the characters can reach each other. So, it seems like it would be better to use the A* Algorithm to determine if there is a path between the two characters.