In the Greek text there are two layers of meaning :
το πνευμα το αλαλον και κωφον εγω σοι επιτασσω [TR] [Mark 9:25]
... the spirit the dumb and deaf I thee command [Literal EGNT (1)]
There is a person before Jesus. And within that person there is a demonic being, by whose presence the person is rendered dumb.
The father only knew the boy was dumb (note the man's words to Jesus). The reason he was dumb was that he was deaf. The deafness made him dumb. It is Jesus who discerns that the boy is deaf.
The boy sees people's mouths move, but he hears no sound. He doesn't know how to make 'sound' for he is not aware of what 'sound' is.
There is a wealth of spiritual meaning in this incident. Without hearing, humanity cannot communicate (with God).
When Jesus rectifies the situation, he speaks - I would suggest - on two levels.
He is communicating with an invisible spirit. He needs not utter any words for he has power over such demonic beings.
So first he addresses the spirit directly, 'the spirit'.
But among men, on earth, in the world, Jesus is manifest and a boy is in front of him and he then addresses what is visible 'the dumb and deaf'.
His spoken words, spoken with a human mouth, are heard in the atmosphere and penetrate the ears of the boy.
And his spiritual power directly affects the invisible demon inside the boy.
Two layers. Two articles.
With the enforced departure of the evil influence, the boy can now hear. And thus, he can begin to learn how to speak.
All of this has spiritual meaning for mankind.
It is my own understanding that the spirit itself is not deaf and dumb, but its presence has that effect on the host that it invades. This is, I understand, a similar kind of thing to a flu virus which does not itself have influenza symptoms.
The virus affects a host and the host suffers the symptoms of the disease carried by the invading species.
By removing all traces of the invader, and by treating the consequent disease, the host will be cured and the symptoms will disappear.
(1) The Englishman's Greek New Testament, literal interlinear translation.