There's a general rule of thumb that you want at least 6 hours between the two workouts of the day, ideal would be more like 8. And on a professional runner lifestyle, there's no reason to not do 8. Now, to the best of my knowledge elites don't typically run at 6am unless they also have a day job, but let's say they go at 7:30 for their morning run, done by 8:30. Fast forward 8 hours and it's 4:30, perfect time to start the second run. Just enough time to get in all the weights/core/plyos/etc afterwards before a nice dinner.