Well you could make a huge map with basemenets on different corners so that they can't be seen from the others. Then you could just teleport to the different basements depending on characters.
On the stairs event make a conditional branch checking for a hero 1, else check for hero 2, and so on and so forth. If hero 1 is there, teleport to basement map, but in the top left corner room. 2 to the bottom corner, or something. I hope that helps.