This topic has been discussed before, here is a link to one of the previous discussions.
http://fastrunningblog.com/forum/index.php/topic,228.0.htmlI personally run and advocate two-a-days for several reasons:
1) Time: Allows me to run higher mileage with other commitments (work, family)
2) Recovery: Early morning runs are typically easy recovery mileage to increase base load safely.
3) Quality: Allows for good quality workouts while increasing mileage safely.
4) Injury prevention: A better way to increase mileage to higher levels safely to avoid injuries from lots of long single runs.
I have been running 2-a-days for a couple of years now and have done so safely. It has allowed me to improve my overall conditioning and race performance. But it does require a commitment and change in schedule to takes time to adapt to.
So, early morning easy runs of 4-10 miles, then quality afternoon runs focused and strength or speed can get you to your improvement you desire. I suggest starting out slow (1-2 days a week) then increasing as your body adapts. Don't forget you still have to do you long runs during the training cycle as well (doubles does not replace the weekly long run).
My initial thoughts, hope this helps.