Elite triathletes don’t often attempt extreme doubles of that sort. The reasons it’s slightly more common in triathlon than in running are partly physiological and partly cultural. Triathlons are mostly swimming and cycling, which are easier to recovery from, and the running is slightly less intense than it is at equivalent distances in running. But I would submit that many elite runners could get away with racing more often than they do. But the culture of the sport is different. Elite runners are trying to shave every second, whereas triathlete are far less focused on the clock. Ultrarunning is similar, and in fact you do see some elite runners racing extreme distances fairly frequently.