I know that I can script it so that the other NPC doesn't run into the room until the first guy is done chanting, but if I do that, there could be a long delay. The problem is that the time is variable for how long it takes for the second NPC to run into the room.
Does anybody know of a way to stop an NPC from saying a line partway through? I suspect not, so I think I'll have to get creative.