A tire manufacturing unit having two tire building drums mounted on respective coaxial and independent half shafts butt connected in a rotary manner so as to define a single continuous shaft supported in a rotary manner on a platform mounted for rotation in successive 180.degree. steps about a horizontal axis; each half shaft having a respective independent driving device, and one of the drums moving, during each rotation step of the platform, between a first and second work position, through a cavity formed in the platform supporting base.