One situation where this is not necessary, is riding on open tundra. If you’re riding where there are no obstructions, nothing to ride around and nothing that you could run into, then riding side-by-side is permissible so long as you are able to keep a safe distance between riders. 50’ to 75’ between riders should be enough room for comfort. Remember that even on the flattest tundra there will always be tussocks that you will need to maneuver around.