Is spellbook building with only a single core set possible?
My wife and i play with the Mage-War Academy core set.
We each have a custom deck. Since the wizard and the beast master have no common type of spell, it's rather easy.
However, should we both want the same spell in 3 copies when the said spell is only available in 1-3 copy, you should photocopy it so that both player has acces to the number they chose. i'd recommend sleeving them, cause... y'know... paper. Since the cards are not drawn, it is of no matter if it's different from others.   

Take special notice of the enchantment cards. They are used as "hidden" information. You should play a completely unrelated card (i recommend using Equipment cards of the OPPOSING wizard) in its stead so that it looks and feel like a normal card. And switching the card with your paper one when it is revealed. Take care to have different "replacement" card for each of your enchants. And write on a sheet of paper the corresponding pair so your opponent cannot accuse you of lying.

Be warned that most gaming groups and tournaments will not let you play with fake cards.