The answer
The Holy Spirit was sent from the Father who is the giver of salvation and sanctification. The Spirit is also sent from the Son who paid the price on the cross so that the Spirit can abide in humanity, and so that the righteousness of Christ may be found in humanity.
St. Ambrose says, “So both the Father and the Spirit sent the Son; the Father sent Him, for it is written: ‘But the Paraclete, the Holy Spirit, Whom the Father will send in My Name’ (John 14:26). The Son sent Him, for He said: ‘But when the Paraclete is come, Whom I will send unto you from the Father, even the Spirit of Truth’ (John 15:26). If, then, the Son and the Spirit send each other, as the Father sends, there is no inferiority of subjection, but a community of power.”870
He also says, “That this same Comforter is also to be sent by the Father He has already taught, saying, ‘But the Comforter, that Holy Spirit, whom the Father will send in My name’ (John 14:26). Behold their unity, inasmuch as whom God the Father sends, the Son sends also”871
Fr. Tadros Yacoub Malaty
Notes
870 St. Ambrose of Milan, On the Holy Spirit 3.1.8 (NPNF II/10:136).
871 St. Ambrose of Milan, Exposition of the Christian Faith 2.9.76 (NPNF II/10:233).