Catch the interview of Wetmore. He himself admits that he would have his athletes doing two runs a day and that getting it all in one run primarily is due to time constraints of classes etc.
I believe in the morning run. Running 70 to 100 miles a week has been shown through experience and trial and error to be the level that college distance runners compete the best at and to do this all in one run would mean running 10 to 14 every single run. A 18 to 22 year old I think would often be fatiguing themselves running this long every day and leave their legs too tired to be fresh for quality workouts and races. But keeping in the mileage range is important for most.
Two runs is the way to go. Plus , maybe more importantly, its the cocnsistent mental reinforcement that you're a RUNNER and that affects all decisions throughout the day to an extent when you hop out of bed first thing, hit the roads, and start the day.
Singles are a hamburger they sell at Wendy's. Two runs are the way to go.