When two high powered spells of two elemental runes are cast at the same time they have a chance to unite and form a united magic attack. United Magic attacks are far stronger then only single rune spell could ever be. Almost every game with a United Magic opinion requires that the final spell of both runes be cast in order to work. The only game that this isn't required is Suikoden V but the both spells must still be of the same level. Also the weaker the level the weaker the spell.