I've seen in various training logs of a lot of professional and semi-pro runners back-to-back hard days. I'd love to hear your experiences integrating this into your training.
Some examples I've seen:
Interval session followed by long run at "sub LT" pace the next day.
Threshold Run followed by a few short(200-500m), fast intervals at max effort with long rest, later in the day.
Many of Canova's athletes, especially late in the season, incorporate 2, sometimes 3 days of intervals or hard running in a row, followed by 1-3 easy days.
Love to hear your experiences, how it may or may not incorporate into your training, and the results you've seen. There's a day or two every few months that I feel like breaking into a tempo run after running a track workout the day before, but otherwise I can't imagine running two consecutive hard workouts on a consistent basis.