Also, when you have several leaders in the same army, you can change the actual leader (or just check the other leaders) by clicking on the little orange arrow at the top of the army/fleet screen...
By the way, a related question....is it of any use having two armies fighting in the same province, each with a leader, or does the highest ranking leader take command of both armies in this case?
For instance if the leader of the first army has a better siege value and the leader of the other one a better shock value, will the the better bonus be used according to the current circumstances (siege, assault, attack by another army...) or will only the bonus of the highest ranking general be used, as it would happen if both were in the same army?