Adam Clarke on 2 Kings 2:9

A double portion of thy spirit be upon me - This in reference to the law, Deu 21:17 : He shall acknowledge the first-born, by giving him a Double Portion of all that he hath-the right of the first-born is his. Elisha considered himself the only child or first-born of Elijah, as the disciples of eminent teachers were called their children; so here he claims a double portion of his spiritual influence, any other disciples coming in for a single share only. Sons of the prophets means no more than the disciples or scholars of the prophets. The original words פי שנים pi shenayim, mean rather two parts, than double the quantity.