Question: how nakul and sahadeva were born?
Answer: Madri mothered Nakul & Sahdev using the same boon. & Pandu was not dead at time of birth of 5 pandavas. It was again Pandu's wish to help Madri bear children of gods that Kunti obliged. Madri called upon twin gods Aswin Kumars and bore Nakul & Sahdev with them.