I recently started a game with two human fighters that I was planning to dual class to a cleric and a magic user when I leveled to 2nd level. When I went to level up (and use the dual class button), it was darkened and unusable. What did I do wrong? Any help?

make sure the strength on both of them are at least 15 and the Int for the mage and Wis for the cleric is 17.