Combined the optimal placement method of triaxial accelerometers based upon the effective independence algorithm(EFI3) and the improved onedimensional effective independence algorithm, the optimal placement method of bidirectional accelerometers for the tower (EFI2) is proposed. The modal matrix on xdirection and ydirection are used as input instead of the uniaxial data. And according to the most linear independence and greater energy distribution of modal shape, EFI2 can optimize the sensor placement of accelerometers on both directions at the same time. On a long span extra highvoltage transmission tower as a case study, the sensor placement in the direction of transversal and longitudinal wires is optimized by the EFI2 method. And with assessment by two methods, it is found that the EFI2 method can optimize the sensor placement of accelerometers on both directions effectively.