Dual-comb sources with equally spaced and low phase noise frequency lines are of great importance for high resolution spectroscopy and metrology. In the terahertz frequency range, electrically pumped semiconductor quantum cascade lasers (QCLs) are suitable candidates for frequency comb and dual-comb operation. Although free running terahertz QCLs can be operated as frequency combs and dual-comb sources, the phase noise originated from the carrier frequency and repetition rate instabilities are relatively high, which hinders the high precision applications. The locking techniques that have been used for a single laser comb can be, in principle, applied to a dual-comb laser source. However, the complete locking of dual-comb lines considerably complicates the implementation of such a system. Here, a method is proposed to stabilize a terahertz QCL dual-comb source by phase locking one dual-comb line. Although only one dual-comb line is locked, it is shown that the phase noise of other dual-comb lines close to the phase locked line is significantly reduced. Finally, it is demonstrated that the terahertz QCL comb without a control of the repetition rate can produce pulsed-type waveforms. The demonstrated approach provides a convenient method to actively stabilize terahertz dual-comb laser sources, which can be further utilized for fast gas sensing and spectroscopy.