I think this was a divine person/witness within the Godhead. There are more examples.
Hagar ran away from Sarai in Genesis 16:
7The angel of the LORD found her by a spring of water in the wilderness, the spring on the way to Shur.
Most versions translate malak Yahweh here as "the angel of the LORD". Brenton Septuagint Translation translated it as "an angel of the Lord".
8And he said, “Hagar, servant of Sarai, where have you come from and where are you going?” She said, “I am fleeing from my mistress Sarai.” 9The angel of the LORD said to her, “Return to your mistress and submit to her.” 10The angel of the LORD also said to her, “I will surely multiply your offspring so that they cannot be numbered for multitude.”
At this point, the angel spoke as first person authoritatively as if he was the LORD. The wording was similar to what the LORD said to Abram in Genesis 13:
16 I will make your offspring as the dust of the earth, so that if one can count the dust of the earth, your offspring also can be counted.
The angel continued:
11And the angel of the LORD said to her, “Behold, you are pregnant and shall bear a son. You shall call his name Ishmael, because the LORD has listened to your affliction.
Here, the language was not in first person. Still, the angel seemed to be a member of the divine Godhead. Similar language is found in Exodus 24:
1Then the Lord said to Moses, “Come up to the Lord, you and Aaron, Nadab and Abihu, and seventy of the elders of Israel. You are to worship at a distance, 2a but Moses alone is to approach the Lord.
Hagar thought the angel was the LORD:
13 So she called the name of the LORD who spoke to her, “You are a God of seeing,” for she said, “Truly here I have seen him who looks after me.” 14Therefore the well was called Beer-lahai-roi; it lies between Kadesh and Bered.
The angel of the LORD here seems to differ from the LORD-Yahweh but he also seems to be a divine person/witness. They seem to be two distinct witnesses in God/Divinity.