When the title said a two lane road, I was assuming one lane in either direction, in wich case it’s courteous to go in single file so cars can pass – a road like this, when traffic is flowing freely, I’m amazed that it’s even a question. Of course it’s both right and legal that they were riding side by side.
A side point as well – pedestrians, horse riders and cyclists have a legal right to use the roads. Drivers only do so under license – drivers have less right to be on the road, not more.