I know there are pro's and con's on the effectiveness of double workouts, so I will get one of the major con's out of the way.
In my opinion, there is no debate, breaking up a long run over two runs isn't as effective as doing the long run in one run. I don't know of too many people that would argue that, BUT there are advantages to doing double workouts.
First, I like doing double workouts because it promotes calorie burning 24/7. So, I will do one or two double workouts so that I can "lean up" for a peak race.
Also, I find that double workouts simulate sore legs at the end of a long race. This helps me mentally to prepare for the final miles of a 100 mike ir 24 hour race.
Lastly, I like double workouts because the second workout can serve as a "shake-out" run to get the legs ready for a key workout the next day or it can be used to loosen up the legs after a morning speed work session.
I know double workouts may not be popular by some, but I find them to be an essential part of my training.