Two (or more) entities need to know about their relations - this is based on your question about using Live loader, if you need to use that tool, your dataset must/should have more context. But Live loader doesn’t support it for now. In that case, I would show you a way to do it via Upsert Block. But I need to know if your dataset has contexts and the Upsert Query would be set based on it. Got it?
This information (the relational context) must be provided in two ways. One by one (in general by application, so the actual user creates the comment and the relation is set or by you - the admin) or via contextual data or “metadata”. That says “This node is related to this ID X123”. So knowing this I can create an Upsert Block just for that.
I’ll go on just a little bit more to be clear.
You have: User and Comments nodes.
But you also need the relational data. The ID from the User and the Comment alone don’t create context.