ever think of having the good characteristics of one schools added to your own well thats where this comes in.
Fire+Ice= DOT attacks of fire and health and defense of ice
Death+Life=Steals of death heals of life.
Myth+Storm= Minions of myth(with a little accuracy) and attack power of storm.
Balance since it is balance can be jointed with any school.(if your wondering any schools can be jointed)
So it could work like this:
If you get up to the fifth trainable spell in your natural school and another school you can go to the joint school of the 2.
Lets take this even further, if you get to the fifth spell in 3 schools you can go to the joint school of the 3.(this is probably never gonna work too many schools or make celestia a school covered world)