The Holy Spirit is One. And Psalm 110 plainly speax of a conversation between TWO PERSONAGES. I'm sure David saw it that way, as JESUS called him a prophet, and referred to what David wrote, which was undoubtedly a vision David saw..  



And Mark14:36  plainly speax of the same kind of conversation between TWO PERSONAGES prior to and leading up to Psalms 110:1 pointing toward that finished work at Calvary, indicating God who is one Spirit  finished the work and was now resting in it.(sitting at the right hand)

Mark 14:36And he said, Abba, Father, all things are possible unto thee; take away this cup from me: nevertheless not what I will, but what thou wilt.

When looking at the revealed conversations of God . In effect as a mystery, God is speaking to God as our one Lord.(Lord said to my Lord.) It seems God can hear His own thoughts and as Christ said in John 11 the fact that He does reveal them in His word was for us to believe, not God who is one Spirit, in whom we believe.

Then they took away the stone from the place where the dead was laid. And Jesus lifted up his eyes, and said, Father, I thank thee that thou hast heard me.And I knew that thou hearest me always: but because of the people which stand by I said it, that they may believe that thou hast sent me. John 11:41-42

Again, God is not greater than God; it’s the position that one took which was greater than the other. The one work of one faith was mutual.

Ephesians 4:5
One Lord, one faith, one baptism,