We investigate the tunability of a T-shaped waveguiding diplexer, based on a two-dimensional square lattice photonic crystal (PhC) composed of silicon rods in air and in liquid crystals (LCs), respectively. For both devices we prove that a complete splitting of the entire input wavelength range into right and left arms respectively can be achieved. Moreover, a tuning of the switching wavelength can be obtained by changing the refractive index of silicon by the thermo-optic effect for PhCs in air, and the refractive index of liquid crystals by the electro-optic effect for PhCs in LCs.