The Apostle Thomas was a Jew, and the Jews (at the time of Jesus) were strict monotheists. They did not tolerate polytheism at all. They knew these Scriptures extremely well:
Exodus 20:3 (ESV): "You shall have no other gods before me.
Deuteronomy 13:1-5 (ESV): "If a prophet or a dreamer of dreams arises among you and gives you a sign or a wonder, and the sign or wonder that he tells you comes to pass, and if he says, 'Let us go after other gods,' which you have not known, 'and let us serve them,'you shall not listen to the words of that prophet or that dreamer of dreams. [...] But that prophet or that dreamer of dreams shall be put to death, because he has taught rebellion against the LORD your God [...]
Psalm 81:9 (ESV): There shall be no strange god among you; you shall not bow down to a foreign god.
Isaiah 43:10-11 (ESV): "You are my witnesses," declares the LORD [Jehovah], "and my servant whom I have chosen, that you may know and believe me and understand that I am he. Before me no god was formed, nor shall there be any after me. I, I am the LORD [Jehovah], and besides me there is no savior.
Deuteronomy 32:39 (ESV): "'See now that I, even I, am he, and there is no god beside me; I kill and I make alive; I wound and I heal; and there is none that can deliver out of my hand.
Deuteronomy 4:35 (ESV): To you it was shown, that you might know that the LORD is God; there is no other besides him.
2 Samuel 7:22 (ESV): Therefore you are great, O LORD [Jehovah] God. For there is none like you, and there is no God besides you, according to all that we have heard with our ears.
1 Kings 8:23 (ESV): and said, "O LORD [Jehovah], God of Israel, there is no God like you, in heaven above or on earth beneath, keeping covenant and showing steadfast love to your servants who walk before you with all their heart,
Isaiah 45:5-6 (ESV): I am the LORD [Jehovah], and there is no other, besides me there is no God; I equip you, though you do not know me, that people may know, from the rising of the sun and from the west, that there is none besides me; I am the LORD [Jehovah], and there is no other.
Isaiah 44:6-8 (ESV): Thus says the LORD [Jehovah], the King of Israel and his Redeemer, the LORD [Jehovah] of hosts: "I am the first and I am the last; besides me there is no god. Who is like me? Let him proclaim it. Let him declare and set it before me, since I appointed an ancient people. Let them declare what is to come, and what will happen. Fear not, nor be afraid; have I not told you from of old and declared it? And you are my witnesses! Is there a God besides me? There is no Rock; I know not any."
Isaiah 45:21-23 (ESV): Declare and present your case; let them take counsel together! Who told this long ago? Who declared it of old? Was it not I, the LORD [Jehovah]? And there is no other god besides me, a righteous God and a Savior; there is none besides me. "Turn to me and be saved, all the ends of the earth! For I am God, and there is no other. By myself I have sworn; from my mouth has gone out in righteousness a word that shall not return: 'To me every knee shall bow, every tongue shall swear allegiance.'
Isaiah 42:8 (ESV): I am the LORD [Jehovah]; that is my name; my glory I give to no other, nor my praise to carved idols.
Isaiah 48:11-13 (ESV): For my own sake, for my own sake, I do it, for how should my name be profaned? My glory I will not give to another. "Listen to me, O Jacob, and Israel, whom I called! I am he; I am the first, and I am the last. My hand laid the foundation of the earth, and my right hand spread out the heavens; when I call to them, they stand forth together.
Hosea 13:4-5 (ESV): But I am the LORD your God from the land of Egypt; you know no God but me, and besides me there is no savior. It was I who knew you in the wilderness, in the land of drought;
Therefore, for a Jew to believe in and serve and worship more than one God was idolatry and blasphemy and was a sin of the highest order.
Saying something like "Oh my God!" or "Oh God!" for a Jew would also have been a sin, "Taking the Name of the Lord in vain." In fact, the Jews had stopped even saying the Divine Name for fear that they would use it in vain.
Thomas said to Jesus "The Lord of me and The God of me" in the Greek (see the Kingdom Interlinear Translation), even though he knew all of the above Scriptures that declare that JEHOVAH is the ONLY GOD for Jews.
Did Thomas believe in two different Gods who were both THE God of him? That would have been polytheism and idolatry. Did Thomas believe in two Gods -- one Almighty and the other a lesser, inferior, created god? That is also polytheism and idolatry.
Or, did Thomas believe that both The Father and The Son are The One True God together, as Jesus Himself taught? Look what Jesus had taught:
Mark 12:29-32 (ESV): Jesus answered, "The most important is, 'Hear, O Israel: The Lord our God, the Lord is one. And you shall love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.' The second is this: 'You shall love your neighbor as yourself.' There is no other commandment greater than these." And the scribe said to him, "You are right, Teacher. You have truly said that he is one, and there is no other besides him.
John 10:29-30 (ESV): My Father ... is greater than all ... I and the Father are one."
John 5:22-23 (ESV): The Father judges no one, but has given all judgment to the Son, that all may honor the Son, just as they honor the Father. Whoever does not honor the Son does not honor the Father who sent him.