They are combined by the same filename and also the tag, but its more complex. The Text for the questlog is also saved in a stringtable file in /game/PillarsOfEternity_Data/data/localised//text/quests. quest files in /game/PillarsOfEternity_Data/data/quests. Every state of the conversation displays the corresponding text from the stringtable file. These Files are combined by the filename and the tag.
These are saved in /game/PillarsOfEternity_Data/data/localised//text/conversations. To each Conversation is a Stringtable file needed, which contains the texts for the NPC and your reactions. conversation files in /game/PillarsOfEternity_Data/data/conversations. The Conversation with the Questgiver or an attached Gameobject (Item, NPC, …) and the Quest. To build valid xml files use the 2 xsd files quest.xsd and conversation.xsd and an xsd validator like this one The Structure of Quests is externalised as xml files.
1.1.5 What can I do besides display text?.