Within my current block of training Coach Z has incorporated a double run day into the mix. This is a common practice for pure runners who are single sport athletes. I believe many triathletes also utilize the double run at various places during the season. It can be a very effective way to boost run volume while limiting the muscular fatigue & damage if done wisely. I have found the most benefit in my own training and coaching of athletes to apply it as a primary longer run done early in the day or morning with specific focus on tempo, race pace or hill work then later in the afternoon or evening a VERY easy shake-out/recovery short run on soft surface & flat terrain. A three to four hour minimum window between runs will allow for recovery. The second run can be very therapuetic if done on grass soccer fields or a trail while not monitoring heart rate, speed, etc. Throwing in some barefoot running if on grass or the infield of a track as part of this embarassingly-slow second run can be fun and good for strengthening the support/stabilizing muscles, ligaments & tendons of the feet. If doing some barefoot running I highly recommend starting in brief increments and slowly growing the amount of time from week to week (start at 2-5 minutes and each week add a minute or two over a block of training).
The double run can be a great way to maximize your time while boosting your run volume in preparation for a key race.
Keep it smooth...