Complex sentence: It had been understood that Margaret should, at the first convenient opportunity, provide the three kingdoms with a king who was to be a kinsman of all the three old dynasties, although in Norway it was specified that she would continue ruling alongside the new king.
Split into simpler sentences: It had been understood that Margaret should, at the first convenient opportunity, provide the three kingdoms with a King who was to be a kinsman of all the three old dynasties. However, in Norway it was specified that she would continue ruling alongside the new King.