Who is the Companion whose testimony is equal to two testimonies, and what does that mean?

Khuzaimah ibn Thabit al-Ansari is the Companion known as Dhul-Shahadatayn (Possessor of Two Testimonies), meaning that his single testimony was equivalent to the testimony of two men. This became apparent when Abu Bakr was compiling the Quran, as no verse was accepted without two just witnesses. When the last part of Surat at-Tawbah was found only with Khuzaimah ibn Thabit, his testimony was accepted. Umar ibn al-Khattab also did the same concerning the verse of stoning. The Prophet, peace be upon him, affirmed that Khuzaimah's testimony was equal to two testimonies, and the Rightly Guided Caliphs accepted this.