No, it is not legal. The agent or attorney in fact (the person given power by the POA) cannot give her rights or authority to another person. However, the principal (person granting the POA) can do this easily by creating a new POA which states that it revokes the old one and gives the power to a new person. So your partner and you can do this, just not the way you did. Only the person who created the POA can change it, and it is done by creating a new POA.