At what point in a training plan should two-a-day sessions be incorporated to maximize performance gains without risking burnout, and what specific types of workouts would be most beneficial to pair together in a single day, considering the delicate balance between intense interval sessions, endurance rides, and active recovery workouts? Should these double days be reserved for specific periods of the training cycle, such as during intense build phases or pre-competition tapering, or can they be effectively integrated throughout the entire season? What are some key indicators that an athlete is ready to handle the increased volume and intensity of two-a-day training, and how can coaches or self-directed athletes adjust the frequency and intensity of these sessions based on individual responses to the increased workload?