welcome at our forum.
I'll try to answer your questions in English language.
Yes it is possible that a hero is equipped with different spells of the same type.
BUT, after all, a hero has just one defens reaction per enemy turn.
If your first hero attack the mage from your example an he uses the shield spell for defense, he can't do anything about a attack from a second hero or creature at the same turn.
On the other Hand a defense spell isn't cast instantly. The player of the mage has to throw a dice zo check if the spell would be succesfull.
Warum muss DSA so kompliziert sein?
Bei D&D würde ein Banbaladin 3W6 Schadenspunkte machen und gut wärs...