Both of these palaces sit on the same axis (approximately facing ESE and opening to the back toward WNW). The cities that grew up around them use this same axis for major thoroughfares that define the urban space. Did this axis have any significance (such as perhaps the direction of the rising sun on a particular date) that attracted builders and the king? Was the Louvre simply sited along the Seine and then Versailles later built to mimic the Louvre's alignment? Or is there some other explanation or meaning to these sitings?
Well, the Louvre and the Bastille were both originally built as military compounds to guard the Seine. Paris was a walled city and had heavy defenses. However, they couldn't build a wall across the Seine, so instead had the military forts at each place where the Seine pierced the defensive wall. So that was the rationale there. I don't know why Versailles was built at that axis.