מלכים ב ב:י פירוש: From David to Destruction, רש״י ורד״ק

You have made a difficult request. It is impossible [for me] to give you more than I possess.⁵According to Rashi, Elisha apparently asked for a double portion of Eliyahu’s spirit. Alternatively, Elisha requested to attain two thirds of Eliyahu’s spirit and for all other prophets together to attain one third. -Radak Or, Elisha requested double the spirit of any other prophet.—Ralbag

You asked something difficult ~ It would have been enough for you if you had asked a just part of my spirit, or even half, but asking for it twice is something difficult, and even though [it is difficult] if you have the spiritual strength to see me as I am taken from you then I will know that from God this will happen, and so you will have what you asked.