I believe that they do 2 sessions done only 4-6 hourts apart.
Without going into too much detail (which I know), I can tell you that their program is built around threshold training. All the brothers do 3-5 workouts a week, but often with two consecutive workouts on the same day. This is mostly threshold (very intermittently based) and a hill workout of around 1Xx200m (where they achieve severe acidosis).
But summed up: threshold intervals, A LOT of it
But this support my thinking about the importance of threshold training, with some anaerobic capacity work.