To address the demand for polarization- and frequency-multiplexed beam control in Reconfigurable Intelligent Surfaces(RIS), a terahertz dual-band, orthogonally polarized, programmable metasurface based on composite unit cells is proposed. By employing an HEMT-based I-shaped dipole with polarization diversity switching, independent control of the two frequency bands is achieved. CST simulations show 180°±20° phase-shifting bandwidths of 36 GHz in the low-frequency band(0.190~0.226 THz) for Y-polarization and 15 GHz in the high-frequency band(0.358~0.373 THz) for X-polarization. Within these bands, the average magnitudes are 0.44 (Y-pol, low band) and 0.53 (X-pol, high band), respectively. Using fractional, block, and convolutional coding, Matlab-based array simulations verify continuous dual-polarized dual-beam scanning(±13.1° to ±81°), multi-beam reconstruction (up to four beams), and single-beam steering. Compared with conventional single-band single-polarization reflectarrays, the proposed dual-band dual-polarization coded RIS offers significant advantages in spectral efficiency and beam-control flexibility, providing a new technological path toward high-capacity, wide-field-of-view, and multifunctional 6G terahertz communication systems.