In reality, AceCalhoon is only for the most part correct. Although the spelling can be blocked with another spelling, a single assistant can not counter-spell a spelling that targets a spell that he is throwing. Although he can accept the reactions, he can not run spelling in the middle of the fireball recitation.
Wizard One tries to throw a fireball.
Player Wizard uses counterspell.
Wizard 1 can not counter-spell because he is throwing a fireball.
Fireball and countererspell both have verbal components. According to the rules of the PHB 203 components, "If you can not provide one or more components of a spell, you can not cast the spell."
You need several assistants for this, or a wizard with a subtle, metamagic spell.