I made a video tutorial for implementing a dialog with multiple NPCs. It was suggested I put it here in the guides section, which makes sense.

Relevant time codes:
-Adding NPCs to dialog editor - 2:34
-Why you can't just add multiple NPCs through the dialog editor and leave it be - 6:46
-The little bit of scripting you need - 8:58
-Set the dialog to start on trigger instead of on clicking NPC - 11:22
-Regions in story editor (I just think they're neat!) - 13:58