At the planning stage of a text, you can put text onto index cards or into the synopsis fields of the outline, which is useful. But there appears to be no easy way to split a card or outline item into two. I think that this would be very useful.
I can see that if there were text in the document, or if the document had subdocuments, that this would become very unwieldy. However, for otherwise empty documents (as would be common during the planning stages), it would be very useful, I think, to have the ⌘K and ⌥⌘K options (or equivalent) available in Corkboard & Outline view, when the cursor is in the synopsis field.
As for merging, I don’t think that would be useful very often.
In effect, this would define a new status for documents: a synopsis but no text or subdocuments. It might be useful to indicate “no text” in the card/outline views; this would have the additional meaning of “splittable”.