"Split" the army and it will be divided in two with each commanding one half.
Alternately, use the "reorganize" button. In the bottom army menu, click the gold arrow right or left until your other leader comes up and then move the forces of your choosing into his command.
Alternately, use the "reorganize" button. In the bottom army menu, click the gold arrow right or left until your other leader comes up and then move the forces of your choosing into his command.