Abstract: An apparatus for grinding both inside surfaces of a concrete joint simultaneously is disclosed. The apparatus includes a proximal grinding wheel having a rotational axis and a grinding implement fixed with a proximal side, and an external drive shaft fixed to the center thereof. A distal grinding wheel is included that has the same rotational axis a grinding implement fixed with a distal side. An internal drive shaft is fixed partially within the external drive shaft and to the center of the distal grinding wheel. The apparatus further includes a means for moving both grinding wheels apart, a source of rotational kinetic energy, and a means for coupling the source of rotational kinetic energy to each drive shaft.