The soul and spirit are two things that are the same.
In John 12:27, Jesus said, "Now is my soul (psuche) troubled", and in John 13:21 it says, "Jesus was troubled in his spirit (pneuma)
Here spirit and soul are used interchangeably. He was troubled in his soul, and he was troubled in his spirit.
Genesis 35:18 (KJV)
18 And it came to pass, as her soul was in departing, (for she died) that she called his name Benoni: but his father called him Benjamin.
James 2:26 (KJV)
26 For as the body without the spirit is dead, so faith without works is dead also.
Here they are used also interchangeably since the body is dead when the soul departs and the body is also dead when the spirit departs.
Soul and Spirit are defined the same in both Hebrew and Greek, and even English. They are synonyms.
English:
soul (sol) noun
1. The animating and vital principle in human beings, credited with the faculties of thought, action, and emotion and often conceived as an immaterial entity.
2. The spiritual nature of human beings, regarded as immortal, separable from the body at death, and susceptible to happiness or misery in a future state.
3. The disembodied spirit of a dead human being; a shade....
5. A human being: "the homes of some nine hundred souls" (Garrison Keillor).
6. The central or integral part; the vital core: "It saddens me that this network . . . may lose its soul, which is after all the quest for news" (M. Kalb).
spir·it (spîr¹ît) noun
1. a. The vital principle or animating force within living beings. b. Incorporeal consciousness....
2. The soul, considered as departing from the body of a person at death.
6. a. The part of a human being associated with the mind, will, and feelings: Though unable to join us today, they are with us in spirit. b. The essential nature of a person or group.
7. A person as characterized by a stated quality: He is a proud spirit.(11)
Hebrew:
SOUL (nephesh):
1) soul, self, life, creature, person, appetite, mind, living being, desire, emotion, passion
1a) that which breathes, the breathing substance or being, soul, the inner being of man
1b) living being
1c) living being (with life in the blood)
1d) the man himself, self, person or individual
1e) seat of the appetites
1f) seat of emotions and passions
SPIRIT (ruach)
1) wind, breath, mind, spirit
1a) breath
1b) wind
1c) spirit (as that which breathes quickly in animation or agitation)
1c1) spirit, animation, vivacity, vigour
1c2) courage
1c3) temper, anger
1c4) impatience, patience
1c5) spirit, disposition (as troubled, bitter, discontented)
1c6) disposition (of various kinds), unaccountable or uncontrollable impulse
1d) spirit (of the living, breathing being in man and animals)
1d1) as gift, preserved by God, God's spirit, departing at death, disembodied being
1e) spirit (as seat of emotion)
1e1) desire
1e2) sorrow, trouble
1f) spirit
1f1) as seat or organ of mental acts
1f2) rarely of the will
1f3) as seat especially of moral character(13)
So in Hebrew "soul" refers to "that which breathes" and to the mind, desire, and emotions.
And "spirit" refers to "that which breathes" and the part of us which experiences emotions and is responsible for "mental acts."
Greek:
Thayer's Greek words for soul (psuche) and spirit (pneuma):
SOUL (psuche):
1) breath
1a) the breath of life
1a1) the vital force which animates the body and shows itself in breathing
1a1a) of animals
1a12) of men
1b) life
1c) that in which there is life
1c1) a living being, a living soul
2) the soul
2a) the seat of the feelings, desires, affections, aversions (our heart, soul etc.)
2b) the (human) soul in so far as it is constituted that by the right use of the aids offered it by God it can attain its highest end and secure eternal blessedness, the soul regarded as a moral being designed for everlasting life
2c) the soul as an essence which differs from the body and is not dissolved by death....
SPIRIT (pneuma)
2) the spirit, i.e. the vital principal by which the body is animated
2a) the rational spirit, the power by which the human being feels, thinks, decides
2b) the soul
3) a spirit, i.e. a simple essence, devoid of all or at least all grosser matter, and possessed of the power of knowing, desiring, deciding, and acting
3a) a life giving spirit
3b) a human soul that has left the body
4) the disposition or influence which fills and governs the soul of any one
4a) the efficient source of any power, affection, emotion, desire, etc.(14)
Thus in Greek "soul" refers to the animating principle which feels, desires, and can attain everlasting life with God.
And "spirit" is also the animating principle which feels, thinks, and decides. Notice once again, the use of the word soul to define spirit (twice in fact: 2b,3b).
Greek:
Strong's Greek words for soul (psuche) and spirit (pneuma):
Spirit
G4151
pneuma
pnyoo'-mah
From G4154; a current of air, that is, breath (blast) or a breeze; by analogy or figuratively a spirit, that is, (human) the rational soul, (by implication) vital principle, mental disposition, etc., or (superhuman) an angel, daemon, or (divine) God, Christ’s spirit, the Holy spirit: - ghost, life, spirit (-ual, -ually), mind. Compare G5590.
Soul
G5590
psuche
psoo-khay'
From G5594; breath, that is, (by implication) spirit, abstractly or concretely (the animal sentient principle only; thus distinguished on the one hand from G4151, which is the rational and immortal soul; and on the other from G2222, which is mere vitality, even of plants: these terms thus exactly correspond respectively to the Hebrew [H5315], [H7307] and [H2416]: - heart (+ -ily), life, mind, soul, + us, + you.
"Summing up, overall the definitions of the English words and lexical entries for the Hebrew and Greek words indicate that "soul" and "spirit" are interchangeable terms, with common characteristics ascribed to both."
Spirit: "by analogy or figuratively a spirit" and "the rational soul"
Soul: "(by implication) spirit" and "the rational and immortal soul"
Same exact meanings.
Unfortunately scripture doesn't provide any reasoning why the two have their own names or why we need two of something that is essentially two of the same thing with no provided differences between them. It's kind of like having a two headed coin, the same on both sides yet one side is not the other side per se'.