Fusion spells should combine two schools and make a card for them to use during a battle. The only way that the player can actually use the Fusion spell is you have to have someone of that school be in the same battle as you and has to cast it on same round. The fusion spells should be for school that don't boost on each other (like there can't be a life and death fusion spell or a ice and fire fusion spell). Or maybe its the other way around and it can be a ice and fire fusion spell.
This is a really neat idea and i support this ! Especially since you need the amulet to properly use your secondary school spells. it would give people a better reason to spend the crowns on their amulet's
But you gave me an idea of fusion spells. What if it works on gardening spells. lets say your plant needs water and sun with the fusion spell you can cast both (for the same energy) But cut the time in haft.
The gardening spells can go up to 2-3 different spells (Wont work with spells that do the same thing that's in different amounts) like to sun spells that have the same effect can't fusion together since they do well the same thing.
For the the level I would suggest 10-30. (if i'm not mistaken that gardening starts at level 10)
(What I thought of you're idea, Mostly questions)
Thanks for the idea but I would like a more detailed version to what you typed (like does it only work for damage spells, healing spells, can they only cast the same amount of pips like hydra fusion with earthquake) and can it work with spells that are attack all fusion with one hitter.
I really love this idea and the possibilities can be endless.
life and 
death fusion spell or a 
ice and 
fire fusion spell). Or maybe its the other way around and it can be a 
