A CNC dual-spindle transmission device is installable on a CNC dual-spindle machining center to receive first and second spindles of the CNC dual-spindle machining center to mount thereto. The CNC dual-spindle machining center includes a base, a machining table movably mounted on the base, and a vertical column erected upright at a rear portion of the base. The CNC dual-spindle transmission device includes a linear rail assembly mounted to the vertical column, a first slide seat slidably mounted to the linear rail assembly and receiving the first spindle to mount thereto, a second slide seat extending across the first slide seat and slidably mounted to the linear rail assembly in a manner of being spaced from the first slide seat to receive the second spindle to mount thereto, and first and second power units mounted to the vertical column to respectively driven the first and second slide seats to move.