Who are the two lords in Psalm 110:1? Are these two lords equal? Are these Lords one and the same? If the LORD and my lord are not equal in the Old Testament, has the meaning of "adoni" in the New Testament changed and "adoni" is now equal with YHWH?
In the first instance, the Hebrew Yahweh is used. A name given by God as His own, and being a third person name form of the verb to be (namely, "I am"). This name refers to God.
"My lord" (Heb. adoni) in Hebrew refers to the king, the lord of the Kingdom. It is in this sense that the mother of the Messiah is called "em adoni" ("mother of my lord [the king]") by Elizabeth (Luke 1:43), itself a royal title denoting queenship.
When David refers to the Messiah, therefore (something conceded by Jews of Jesus' day), and calls him "my lord," he means that he is the king of the king himself - that is to say, greater than God's highest authority on earth.
The degree or nature of the lordship is not specified by the word alone - it is only implied that the lord here mentioned is King David's lord, and in this light Jesus uses this passage to prove the divinity of the Messiah, or, at least, His superiority to Davidic Kings - God's highest ordained authority, acting in His place.
How can the future Messiah be David's "lord," if He isn't pre-existent - isn't God - isn't more father to David than son? "How does David then call thim lord?" (Mt. 22:43).
The equality is not stated definitely, but implied strongly by virtue of the necessary identity of the lord spoken of, namely, the Messiah, God incarnate (John 1:1, 11-14; Rev. 19:13).
No, they are not equal. One is Yahweh, the other is not. There is only one Yahweh as David well understood. Yahweh is God, He is the one revealing what will happen when the 'Messiah' arrives to do all these things and to be the one who is foreknown from the beginning.
1 Pet 1:20 For He was foreknown before the foundation of the world, but has appeared in these last times for the sake of you
Acts 2:36 "Therefore let all the house of Israel know for certain that God has made Him both Lord and Christ-- this Jesus whom you crucified."
Secondly, the word for Lord means master - a word never used for Yahweh. So not only does this make them not equal, it states that the 'Lord' is NOT God.