Part of the preliminary morning service discusses the construction of the kiyor (the laver), from which the Kohanim would wash their hands and feet in the Temple. The Talmud in Zevachim, cited by Rashi, explains that the kohanim washed their hands and feet simultaneously. That is, the kohein placed his right hand on his right foot and his left hand on his left foot. Then, the water was poured on them all at once.