A mechanical device is comprised of a master wheel assembly, one or multiple slave wheel assemblies, a control rod, a shaft assembly, as well as multiple pairs of cord holders and cords. The master and slave wheel assemblies are connected by a shaft assembly and are mounted at the edge of a top slat of a conventional Venetian blind. The master wheel assembly is driven by a control rod and provides rotational power to slave wheel assemblies through the shaft assembly. One end of the paired cords is tightened to the master and slave wheel assemblies, and the other end is tightened, with cord holders, to a user-selected slat that divides the Venetian blind into an upper and a lower region. Manual manipulating the control rod causes the master and slave wheel assemblies to make synchronic bi-directional tilting of slats in lower region of the Venetian blind without interfering operation of the Venetian blind. Combining with operation of slat-adjusting system of Venetian blind, this device enables selective control of slat tilting in lower and upper regions of Venetian blind for privacy and maximum natural lighting adjustments.