A drilling assembly for drilling holes in concrete structures. The assembly includes a base assembly defining a longitudinal axis and adapted to support at least two concrete structures, for example, on turntables positioned in spaced apart relation along the longitudinal axis. A drilling machine is movably mounted to the base assembly between the turntables for movement along the longitudinal axis, and includes a drill assembly having first and second drill bits each substantially aligned along the longitudinal axis and facing substantially opposite directions. In this manner, the drilling machine includes an in-line, dual drill bit assembly which allows the machine to move along a single horizontal axis for both drilling operations and for traversing between the turntables. Advantageously, the present assembly combines the traverse and drilling axes of the drilling machine into a single horizontal axis, thus eliminating the need to traverse the machine between a pair of turntables along a first horizontal axis and to perform drilling operations along a second horizontal axis.