
Well, Winston Churchill, who was a Member of the British Grove of Druids asked all British Mystics, Witches and Druids to extend a protective charm over the British Isles during 1940. We won the Battle of Britain, and the German Invasion never came. So yes, charms and spells can work.
I see, two opposing forces of magical will. Well I suppose the winds of fate would play a part along with the light in the hearts of the players. If magical will is as strong as god's will then a touch of luck is needed. If it is an uneven contest like David and Goliath, then mabe the enchanted pebble can make all the difference.
Then you are saying the two make a null effect.
What remains unanswered is the fairness of such, assuming that you believe that what you invoke works, since if a player or team trains hard and does all they should to win - should someone who charms the other team just because they can be actually be doing ill will?
One could argue that good training in itself is magical, the purpose, the effort and the endeavor is holy will. If a sportsman believes in his victory, has foretold it in his mind, has a passion for overcoming the other team, this is a massive force of will.
Just watch the Welsh Rugby team singing their National Anthem at Cardiff before a game with England.