Two substrate integrated step horn antennas sharing the same volume in the millimeter-wave band are proposed in this paper. The step horns are implemented using the substrate integrated waveguide technology and the multiple layer printed circuit board (PCB) technique. Two step horns radiate from their individual aperture with independent feedings, but are integrated in the same substrate cube. After elaborate designs, both the two horns operate in a wide band from 24 to 30 GHz with the reflection coefficients below -10 dB. The two perpendicular radiating aperture on the top and front, contribute together to a radiation in the 0-plane, resulting in an increased space coverage efficiency. Moreover, a linear broadside array with eight antenna units is investigated to obtain the beam scanning property. Simulation results demonstrate that the radiation beams of the eight-unit array in both directions can steer continuously from -45° to +45° with scanning loss less than 3 dB. The antenna unit and array proposed in this paper are promising for millimeter-wave communication systems on mobile devices.