I want to know the answer to the following question:
Who says with certainty that the cross was made out of two beams?
All the background I'm provided with is that "Justin Martyr (100–165) explicitly says the cross of Christ was of two-beam shape: "That lamb which was commanded to be wholly roasted was a symbol of the suffering of the cross which Christ would undergo.". Is that enough to answer the question?
Answer: yes