In your own Law it is written that the testimony of two men is valid.
“your” law = sets Jesus apart from the Pharisees. The law also applied to them in a completely different manner than it did to Jesus. They were the receivers of the law and He was the Giver.
They made their appeal to the Law and Jesus used the very basis for their appeal to show them their error. Jesus always meets us where we are and speaks on the level of our understanding.
Jesus intentionally misstates the Law (Deut 17:6, 19:15) when He says “men” and the scripture says “witnesses.” His point is that if the Law accepts the testimony of 2 “men,” how much more should the testimony of God and His Son be accepted?
Deut 17:6 On the testimony of two or three witnesses a man shall be put to death, but no-one shall be put to death on the testimony of only one witness.
This particular law was given because men lie and nobody should be condemned upon the lie of one individual. It is less likely for 2 or 3 to give the same lie, but it still happens (1 Kings 21:7-11), which again shows the fallacy of trusting in our own decisions.
1 Kings 21:7-11 Jezebel his wife said, “Is this how you act as king over Israel? Get up and eat! Cheer up. I’ll get you the vineyard of Naboth the Jezreelite.” So she wrote letters in Ahab’s name, placed his seal on them, and sent them to the elders and nobles who lived in Naboth’s city with him. In those letters she wrote: “Proclaim a day of fasting and seat Naboth in a prominent place among the people. But seat two scoundrels opposite him and have them testify that he has cursed both God and the king. Then take him out and stone him to death.” So the elders and nobles who lived in Naboth’s city did as Jezebel directed in the letters she had written to them.