Normally, when people fight Atheon, they have their "away" team call out which side they're on. This is actually completely unnecessary, as it can be solved from the outside with a simple glance at the minimap. As the "away" team ports away, you will notice that their dots, or arrows, now, on the minimap are closer to either left or right. That is the portal which you should open. It really does cause a lot less confusion within the vault if you don't have that extra chatter of "green!" "red!" "left!" "Mars!" etc.