6G/beyond 5G technologies should meet the requirement for both high speed and high security. We have focused on terahertz (THz) waves for high-speed transmission, and by photomixing, we have demonstrated power enhancement and beam steering. In terms of the security of wireless transmission, physical-layer security becomes an important issue. We have proposed a novel wireless communication system based on beam steering and coherent detection. In this system, two different data encoded from original data on two THz beams are radiated from two different locations and are overlapped at a target area by beam steering. Then, a receiver at the area implements logical AND operation between the two data as a result of coherent detection, which deduces the original data. This time, as a feasibility confirmation, we performed logical AND operation between two data on different lightwaves and measured the bit error rate of the deduced original data. Experimental results showed that the original data is deduced exactly from the two encoded data when the amplitude ratio between the two data is smaller than 5. This indicates that the acceptable ratio between the distances from two different transmitters would be up to 5. Experimental results also showed that the bit error rate suddenly deteriorates when the time slots of the two data are shifted by 0.7 bits This indicates that the receivable area is limited in several centimeters square for a 1-Gbit/s data.